Because there’s a lot of chemistry vocabulary out there, I’ve decided to break it down into several pages so you don’t have to scroll so much. This page has vocabulary words that start with the letters A-D, as I’m sure you probably already figured out.
Incidentally, if you see an asterisk (*) after the definition, there will be more information about the topic at the bottom of the page when you scroll down. I’ve included what I think is most important in the main definition, but figured that some of you might need a bit of supplemental information. (Updated 3/2/23)
Definitions:
Abegg’s rule: More a guideline than a solid rule, Abegg’s rule states that the difference between the maximum and minimum possible oxidation states for an element is usually eight. For example, the extremes of oxidation state for sulfur are +6 and -2, the difference between which is 8.
ab initio: Meaning “from first principles”, this refers to calculations in chemical modelling which assume only the basic laws of science and no additional conjectures.
ablation: When the surface of something is removed in a purely surface phenomenon (ie: when something is sandblasted or when a laser blows the surface of something away with the application of energy).
abrasive: A material that, when rubbed on another material, wears away the surface of the material it is in contact with. For example, the rough part of sandpaper is an abrasive because its purpose is to remove imperfections in wood.
absolute temperature scale: An absolute temperature scale is one in which “0” is equal to absolute zero. The main example we use is the Kelvin (K) scale, which is based on the Celsius scale.*
absolute zero: The lowest temperature possible, because particles are essentially stationary. Absolute zero is defined as 0 K, or -273.15 degrees Celsius.*
absorption: When something is soaked up by another. For example, water is absorbed by a sponge.*
absorption spectrum: If you shine light at a sample, some of the energies of light will be absorbed when they cause electrons in the sample to jump from the ground to excited state. The light that passes through the sample will lack these colors of light, allowing the observer to identify the compound.
accuracy: This is a measure of how close a measurement is to the actual value of the thing being measured. For example, if I were to measure my weight as 105 kg, that would be an accurate measurement because I really do weigh that much.*
acetic acid: Though it’s not really a vocabulary term, it’s a common enough compound that I think I’ll mention it anyway. Acetic acid (CH3COOH), also known as ethanoic acid, is the acid you’ll find in vinegar.
achiral: Not chiral. Which is to say that the structure of the compound is superimposable on its mirror image.
acid: Something that gives off H+ ions in water and has a pH less than 7. Strong acids are those that ionize completely (or nearly so) and weak acids do not.*
acid anhydride: An oxide that forms an acid when you stick it in water. For example, sulfur trioxide (SO3) turns into sulfuric acid (H2SO4) when it is combined with water.
acid dissociation constant (Ka): This is basically a measurement of the strength of an acid, where larger values correspond to stronger acids. Here’s a good explanation of how to calculate it: link.
actinides: The elements 89-102 at the bottom of the periodic table in the 5f block. These elements are sometimes called actinoids.
activated complex: When you do a chemical reaction, the reagents hit each other and then [something] and then they bounce away in the form of products. That [something] that’s halfway between the reagents and products, where everything is half-done, is the activated complex. It’s also called the transition state, if you’d rather say that.*
activation energy (Ea): The amount of energy needed for a reaction to take place. As a rule, chemical reaction rates depend entirely on the activation energy of the reaction, with low activation energies corresponding to very fast reactions. An example of a reaction with a low activation energy is the combustion of gasoline, because a little spark makes the reaction take place.
active metal: A metal with an extremely low ionization energy. This causes the atom to very easily form cations in reactions.
activity: The “effective concentration” of a compound in solution. Here’s the idea: In a perfect world, all of the molecules of one compound in solution will be perfectly able to react with all of the molecules of another compound. However, because the world is not ideal (i.e. there are no solutions that actually do this), the activity of a solution is not the concentration of a compound, but rather how concentrated the compound acts in solution. For ideal solutions, the activity is equal to the actual concentration. This concept, incidentally, also applies for ideal gases and their partial pressures, but in this case we don’t call it the “activity” of the gas but rather the “fugacity.”
activity series: When elements are arranged in order of how much they react with water and acids. These are important in electrochemical reactions, because they help you to figure out which material will oxidize which. Also called a reactivity series.
actual yield: When you do a chemical reaction, this is the amount of stuff (in grams) that you have sitting at the bottom of your flask.
acyclic: Refers to an organic compound in which the atoms don’t form a ring.
acyl group: A compound which is similar to a carboxylic acid, but in which the -OH has been replaced with a halogen atom. The general formula of an acyl group is R-COX, where X is the halogen atom.
addition reaction: A reaction where atoms add to a carbon-carbon multiple bond (e.g. hydrogenation reactions).*
adsorption: When one substance sticks to the surface of another one. Activated charcoal is good at doing this when it purifies water.*
alcohol: An organic molecule with an -OH group. Additionally, the term “alcohol” is typically used as shorthand for “ethanol”, which is the type of alcohol you can buy in bars and liquor stores.* (Link)
aldehyde: An organic molecule with a -COH group.* (Link)
alkali metals: The elements in group 1 of the periodic table. They are the most reactive group of metals and are strong reducing agents.
alkaline: Used to describe a basic solution.
alkaline battery: A battery containing KOH as the electrolyte. It’s called an alkaline battery because KOH is a base.
alkaline earth metals: The elements in group 2 of the periodic table. They are less reactive than alkali metals, but still good reducing agents.
alkane: An organic molecule that has only single carbon-carbon bonds.* (Link)
alkene: An organic molecule that contains at least one carbon-carbon double bond. Also called an olefin. (Link)
alkyl group: An alkane in which one of the hydrogen atoms is replaced with a halogen atom. Chloromethane is a very simple example.
alkyne: An organic molecule that contains at least one carbon-carbon triple bond. (Link)
allotropes: The different forms of an element that may sometimes exist in the same state. For example, carbon can exist as both diamond and graphite when solid, so diamond and graphite are both said to be allotropes of carbon.*
alloy: A mixture of a metal and something else. Typically, the something else (which may or may not be a metals) is chosen to give the parent metal some desired set of characteristics.* (Link)
α-carbon: In organic chemistry, the carbon that’s right next to the functional group of interest. This is usually done with carbonyl groups (C=O), where the alpha carbon is the one that’s connected to the carbon atom of the carbonyl group.
α-hydrogen: A hydrogen atom that’s located on an alpha carbon atom (see above).
alpha particle (α): A helium-4 nucleus that is ejected as radiation from another element. This has the effect of decreasing the atomic mass of the original atom by 4 amu and decreasing the atomic number by two.* (Link)
amalgam: It’s an alloy that contains mercury. Amalgams were used as dental fillings in the past, but people now think they’ll cause mercury poisoning so they use ceramics instead. The FDA and the American Dental Association both say that these fillings are safe, but people who like to make money by scaring people on TV have convinced the general public otherwise.
amine: An organic molecule that contains -NR₂ functionality. These compounds act as Lewis bases in reactions, and frequently smell very bad. (Link)
amino acid: Organic compounds that contain both amine and carboxylic acid functionality.* (Link)
amorphous solid: A solid without any particular structure or long-range order. This is contrasted with a crystalline solid, which has a very well-defined structure. Examples of amorphous solids include things like plastic and rubber.
ampere (amp): The SI unit of electrical current. 1 amp = 1 coulomb per sec.
amphiprotic: When something can act as both an acid and a base (depending on its environment). Amino acids are amphiprotic because they have a basic amino group and an acidic carboxylic acid group. (Link)
amphipathic: Compounds with a part that dissolves in water and another part that doesn’t. These things tend to make little membranes that separate oil and water, such as cell membranes and micelles. Soap is a really common amphipathic material.
amphoteric: Pretty much the same thing as amphiprotic.*
analytical chemistry: The branch of chemistry that concerns itself with the identification of unknown compounds (called analytes) and determinations of purity in known materials. Spectroscopy, spectrometry, and titrations are used to do this (among other things).
angstrom (Å): 10−10 meters. This is not an SI unit, though is in common use. (Link)
anion: An atom or group of atoms that have negative charge.
anode: The electrode where oxidation occurs (i.e. where electrons are lost by a substance). In metal anodes, this results in the neutral metal being oxidized into an ion, causing it to dissolve.
antibonding orbital: A molecular orbital that’s higher in energy than the atomic orbitals that created it, making the bonding interaction between the atoms weaker. When indicating an antibonding orbital, you write a little asterisk over it (example: π* orbital).
aqua regia: A mix of 1/3 nitric acid and 2/3 hydrochloric acid. It dissolves gold and other hard-to-dissolve things.
aromatic: Refers to a compound related to benzene, in which there are alternating single and double bonds (usually in a six-membered ring). The total number of delocalized electrons in an aromatic system is denoted as (4n +2), in which n is some integer. For example, if you have 4 conjugated electrons (such as in cyclobutadiene), the system is not aromatic, but benzene with its 6 conjugated electrons, is. Aromaticity confers unusual stability to a molecule, as is the case in benzene vs. the theoretical cyclohexatriene.
artificial transmutation: When one element is changed into another by hitting it with subatomic particles or atoms of other elements. This is how the heaviest elements are made.
aqueous: When something has been dissolved in water.
atmosphere (atm): The average pressure of the earth’s air at sea level. Defined as 101.325 kPa.
atom: The smallest unit of matter that chemists can mess with. Put another way, the smallest chunk you can divide an element into and still have it be the same element.
atomic mass unit (amu): This is 1/12 the mass of a single atom of carbon-12. For all intents and purposes, protons and neutrons both weigh 1 amu.*
atomic number: The number of protons in the nucleus of an element. It is the atomic number that determines which element is present. Often denoted by the letter Z.
atomic orbital: Where the electron in an atom lives. These orbitals have three dimensional shapes and are described by mathematical relationships called wavefunctions.
atomic radius: A measure of how big an atom is (just as the radius of a circle is a measure of how big it is.)*
atomic solid: Also called a “covalent network solid”, this is a giant lattice of atoms that are all bonded covalently to one another. This is present in diamonds, quartz, silicon, and lots of other stuf.
aufbau principle: Every time you add another electron to an atom, it goes into the orbital with the next highest amount of energy. This is why electron configurations are so boring – they are all essentially the same thing with extra electrons added into them.*
autoignition temperature: The temperature at which a compound will spontaneously ignite under normal atmospheric conditions. A good rule of thumb: If you’re heating a flammable liquid and it starts to smoke, you’re nearing the autoignition temperatrure and should probably stop.
autoionization: When a molecule breaks into ions on its own. The best example is the autoionization of water, which breaks into H+ and OH-.
Avogadro’s law: All gases contain the same number of particles (atoms or molecules) per liter if their pressures and temperatures are the same. This law allows us to treat all gases as being entirely equal when doing gas law calculations, which saves us a lot of time.*
azeotrope: A mixture of liquids that can no longer be separated by distillation. For example, a 95% ethanol/5% water solution can’t be separated by distillation.
background radiation: Normal radiation that you’re exposed to by the environment. Caused by small amounts of radioactive elements, as well as cosmic rays from space.
band gap: The energy difference between the top of the valence band of electrons and the bottom of the conducting band of electrons in a material. Materials with large band gaps are insulators, materials with small band gaps are semiconductors, and conductors have overlapping bands (and no band gap).
band of stability: When the number of protons of isotopes is plotted against their numbers of neutrons, there is a stable area (the band of stability) which consists of stable nuclides.
barometer: A device for measuring atmospheric pressure. A related device, called a manometer, uses a column of liquid to measure gas pressure in general.
base anhydride: An oxide that forms a base when water is added. Calcium oxide, for instance, forms calcium hydroxide when you stick it in water.
base: There are three definitions (see the notes below), but the one that’s most commonly useful to us is the Arrhenius definition that bases give off hydroxide (OH-) ions when you put them in water.* Strong bases dissociate completely in water, while weak ones do not.
battery: A device in which electrical energy is stored due to the presence of chemicals that have not yet reacted. When the circuit between the anode and cathode is closed, these reactions can proceed, causing electrons to travel from the anode to the cathode.
benzene: If you look at the little icon for this website, that’s benzene. Though it seems that it would make more sense to call it “cyclohexatriene”, it turns out that, as an aromatic compound, the double bonds in the ring are conjugated, with delocalized electrons moving around the entire ring. This delocalization gives benzene (and other aromatic compounds) more stability than one might expect.
beta particle (β): The term for an electron given off during a radioactive process. Though the loss of a beta particle doesn’t change the mass of the isotope that’s decaying, it does increase the atomic number by one. This sort of decay is also referred to as “beta-minus” decay.*
binary compound: A compound that contains only two elements. Examples include calcium oxide (CaO) and water. Binary acids are acids in which hydrogen is bonded to a single other element (as in HF or HBr).
binding energy: The amount of energy that holds the protons and neutrons in the nucleus of an atom together.*
boiling point: The temperature at which the vapor pressure of a liquid is the same as the ambient gas pressure. When this occurs, the liquid begins to spontaneously vaporize. The normal boiling point is the temperature at which this occurs at a pressure of 1 atm.
bomb calorimeter: A device used to measure the energy given off during some chemical process -this practice is calorimetry.
bond dissociation energy: The amount of energy it takes to break one mole of a specific chemical bond.*
bond energy: The energy it takes to break one mole of a chemical compound into its constituent atoms.*
bond length: The distance between the nuclei of two bonded atoms. Typical bond lengths for atoms are on the order of 10−10 meters, which is a distance usually referred to as the angstrom (Å).
bond order: A measure of the bonding stability between two atoms. Equal to the number of electrons in bonding orbitals minus the number of electrons in antibonding orbitals, divided by two.
bonding orbital: A molecular orbital that’s lower in energy (i.e. more stable) than the orbitals that have overlapped to create it.
Boyle’s law: The volume of a gas varies inversely with pressure. Or put another way, if you push real hard on a gas, it gets smaller.
Brønsted-Lowry acid/base: A Brønsted-Lowry acid gives off H+ ions in water, and a base absorbs those H+ ions. In this definition, it’s not right to say that a compound is inherently an acid – it can only be an acid during some process in which it reacts with a base. Though, admittedly, some compounds are nearly always acids (HCl, for example).
Brønsted-Lowry conjugate acid-base pair: When a Brønsted-Lowry acid loses a proton (H+), the resulting compound is its conjugate base. Likewise, when a base gains a proton, the resulting compound is its conjugate acid.
buffer: A solution that resists changes in pH if an acid or base is added. Buffers consist of weak acids and their conjugate bases (acetic acid/sodium acetate, for example).*
bumping: If you’re heating a liquid, sometimes it just heats and heats until it suddenly starts boiling like crazy and overflows the container it’s in. That’s bumping. If you want to avoid bumping, try using boiling chips or stick a wooden stick into the beaker.
calorimetry: The study of heat flow during a process. Calorimetery is commonly used to determine energy changes resulting from combustion, formation, or reaction.*
carboxylic acid: An organic compound with -COOH functionality. Acetic acid is the most famous, though there are a ton of these guys out there.
catalyst: A substance that speeds up a reaction without being used up. This occurs when a new reaction pathway with a lower activation energy is substituted for the original one.*
cathode: The electrode at which reduction occurs (i.e. the electrode that gains electrons).
cation: An atom or group of atoms that have positive charge.
centrifuge: A device you use to spin stuff really fast. The point of this is to separate these things by density. If you’ve got a colloid and want it to be just a bunch of solid in a liquid, or just have a really stable suspension that’s’ taking forever to separate, you might want to try throwing it in a centrifuge.
chain reaction: A reaction in which the products from one step of the reaction provide the reagents for the next. Chemical chain reactions often have initiation, propagation, and termination steps, while nuclear chain reactions occur when one process starts another.
Charles’s law: The volume of a gas is direction proportional to its temperature. In other words, if you make a gas hot, it gets bigger.
chemical equation: A recipe that tells you how to do a chemical reaction.*
chemical properties: Properties that describe the ability of something to undergo a chemical change (to turn from one chemical into another). For example, one chemical property is “flammability”, because it describes the ability of something to burn.
chirality: A molecule is chiral if it has a nonsuperimposable mirror image. To imagine what this would look like, examine your hands. Though they are clearly related to one another, you can’t put them together such that they exactly overlap because the fingers are on opposite sides. For this reason, chirality is sometimes described as “handedness.”
chromatography: When you separate a mixture of compounds by examining how they move through some stationary material. This is done by dissolving them into a moving stationary phase – depending on their properties, one will tend to spend more time stuck to the stationary phase than the other, causing it to take longer to pass through.*
coagulation: When the particles in a colloid are forced to settle out through the addition of some other material. This other material is called a flocculant, and in this context, coagulation is referred to as flocculation.
colligative property: Any property of a solution that changes as the concentration of the solution changes. Examples are boiling point, melting point, and osmotic pressure.*
colloid: When little tiny particles of stuff are permanently suspended in a liquid. This occurs because the particles are so tiny that the solvent particles hit them hard enough to keep them from sinking.
colorimeter: A device that determines the concentration of a solution by shining light through it and seeing how much of it gets absorbed.
combustion: When a compound reacts with oxygen to form water, heat, and carbon dioxide.
common ion effect: When the equilibrium position of a process is changed due to the addition of another compound that contains one of the ions involved in the equilibrium. For example, if you have something that’s pretty insoluble like calcium hydroxide, you can make it even less soluble by adding some other base.
complex ion: An ion in which a central atom (usually a transition metal) is surrounded by a bunch of little molecules such as water or ammonia (called ligands). More info: Link.
concentration: A measure of how much stuff (solute) is dissolved in a liquid (solvent) to form a solution. Common units include molarity (moles of solute/L of solution), molality (moles of solute / kg of solvent), and when people say “hella strong.”
condensation: When a gas reforms a liquid. This occurs on your bathroom mirror when you take a shower and on the side of a glass of soda on a hot day.*
conductance: A measurement of how much electricity can flow through an object. The conductance of a material is the inverse of the resistance, and is equal to the current (I) divided by the voltage (V) in the circuit.
conjugate acid/base: In a Brønsted-Lowry acid-base reaction, a conjugate acid is formed when a base absorbs a hydronium ion (H+). Alternatively, a Brønsted-Lowry conjugate base is formed when an acid loses a hydronium ion. As a result, if hydrochloric acid reacts with water, you get the reaction: HCl + H2O -> Cl- + H3O+, where HCl is an acid and Cl- is its conjugate base, while water is the base and the hydronium ion (which can also be shown as H3O+) is its conjugate acid.*
continuous spectrum: A spectrum that gives off all colors of light, like a rainbow. This is in contrast to a line spectrum, which gives off only certain colors/energies of light.*
coordination complex: This is when you have a metal ion surrounded by a bunch of ligands. They’re usually pretty big.
covalent bond: A chemical bond formed when two atoms share an electron pair. This results in the formations of molecules or larger networked solids, depending on what is bonding and the conditions under which the bonds are formed. It is very common to see single bonds, double bonds (where two electron pairs are shared), and triple bonds (where three electron pairs are shared) between two atoms.*
critical mass: The minimum amount of radioactive material needed for a self-sustaining nuclear reaction.*
critical point: The end point of the liquid-vapor line on a phase diagram. Past the critical point, the material becomes a supercritical fluid, which has properties somewhere between those of a liquid and gas.
crystal lattice: The arrangement of atoms or ions in a solid. Crystal lattices consist of repeated units called “unit cells“. The term crystal lattice is often just replaced by “lattice” for some uses (e.g. “lattice energy”).
crystal: A chunk of a material that has a regular, lattice-containing form (typically either a network atomic solid or an ionic compound).
Dalton’s law of partial pressure: The total pressure of a mixture of gases is equal to the sums of the partial pressures of each gas in the mixture.*
decomposition: When one compound falls apart into several smaller ones during a chemical reaction.*
degenerate: Things are said to be degenerate of they have the same amount of energy – this term is most commonly used when describing orbital energies and in punk rock.
delocalization: When electrons can move around all over a molecule due to molecular orbital overlap. This occurs in conjugated hydrocarbons such as benzene, which contains alternating single and double bonds.*
denature: When the 3-D structure of a protein breaks down for various reasons (pH, heat), causing it to become nonfunctional.*
deprotonation: The loss of H+ from a compound.
diffusion: When particles move from areas of high concentration to those of low concentration. For example, if you open a bottle of ammonia at one side of a room, the smell will travel to the other side. (Note: Don’t mix this up with effusion).
dilution: When solvent is added to a solution to make the solution less concentrated. Essentially the same thing as “watering it down.”
dipole moment: When you’ve got a polar molecule (a molecule in which there is a partial separation of charge), the degree to which the molecule is charge-unbalanced is called its dipole moment. Link.
dipole-dipole force: When molecules with permanent dipoles (i.e. polar molecules) tend to stick to each other because the partially-positive end of one molecule is attracted to the partially-negative end of the other. Link.
dissociation: When one thing is pulled apart from another. Examples include bond dissociation (when a bond breaks) and dissolving in water (in which a cation and anion or covalent molecules are separated from each other).
distillation: If you have two (or more) liquids that are all mixed together, you can gently heat the mixture, causing one of the liquids to boil before the others. This allows you to separate the mixture, one component at a time.*
double-displacement reaction (double-replacement reaction): These reactions occur when the cations of two ionic compounds switch places with each other to form two new ionic compounds. Such reactions only occur when the following conditions are met: Both compounds are soluble in water, and only one of the products is soluble.
Footnotes for the vocabulary above:
absolute temperature scale: The reason “0” is defined as absolute zero is so that the theoretical minimum amount of energy an object can have (the zero point energy) corresponds to a number that indicates an absolute minimum. Another common absolute scale is the Rankine scale, in which one degree Rankine is equal to one degree Fahrenheit.
absolute zero: Essentially, but not completely, stationary. At absolute zero, there is still some molecular motion which gives a nonzero amount of energy called the “zero point energy.”
absorption: To see how this differs from adsorption, check out the adsorption definition below.
accuracy: For a measurement to be accurate, it also has to be precise. You also have some difficulty in determining accuracy in the real world, because you need to already know the value of something before you can determine the accuracy of the measurement. As a result, we use standards with very clearly known values to calibrate the equipment we use. And after that, we hope and pray that the equipment doesn’t break.
acid: The definition given here is both the Arrhenius and Brønsted-Lowry definition of an acid, and is good for about 99% of the stuff that a first year chemistry student will actually need to know in the lab. However, it should be noted that the Lewis definition of an acid defines it as an electron acceptor. Though this definition is just an expanded version of the other two, it’s a little confusing so we usually just stick with the proton one.
activated complex: The activated complex is at the top of the potential energy curve for the reaction, indicating that from this point the reaction can either proceed in the forward direction to make products or return to the original reagents. Either way, an activated complex/transition state is not at all stable and usually cannot be directly observed.
addition reactions: This is an incredibly incomplete definition, but it’s about the best I can do with a couple of sentences. If you’re interested in learning more, check this out: link.
adsorption: It’s easy to mix this up with absorption, which is a term you’re more familiar with. The best way to describe the difference is to say that absorption is like when a sponge soaks up water, while adsorption is when your clothes smell like cigarette smoke after you go bowling. One is a surface phenomenon (adsorption), while the other involves something being soaked (absorption).
alcohol: Please note that most alcohols will kill you if you drink them, and that lab grade ethanol will also kill you because they put chemicals into it to keep you from drinking it. Though I don’t suggest that you drink any alcohol, if you do, make sure to only drink alcohol that’s specifically labeled for human consumption (link, link)!
aldehyde: Aldehydes are kind of annoying because the names that people actually use for them are different than the official IUPAC names. For example, if you ask for “ethanal”, chemists will understand that you’re a nonchemist who would like to borrow some acetaldehyde.
alkane: Though most organic compounds contain a C-C bond somewhere, only molecules that contain only single C-C bonds are properly called alkanes.
allotropes: And yes, I know that fullerenes and graphenes are also allotropes of carbon, but I didn’t want to make it too confusing. And I’m sure there are more, but I’m not looking them up because I don’t want to.
alloy: Alloys can be either interstitial alloys (where small atoms are located in the spaces between big ones) or substitutional alloys (where atoms of similar size take the place of the parent metal atoms). Or both.
amino acids: Though the definition is clear, actually remembering what’s up with amino acids is a little more annoying. Life on earth is known to use 23 amino acids in protein synthesis, while eukaryotes only use 21. And only 20 can actually be encoded for in DNA, with the others being generated by methods I can’t figure out. And humans can only synthesize 11 of them. And the stereochemistry of each acid makes a big difference in whether it’s useful for anything. You can read more here: link.
amphoteric: But not exactly the same. Amphiprotic materials either donate or accept H+ ions, while amphoteric compounds can act as acids or bases by doing pretty much anything they want, as is the case with metal oxides. Read more here: link.
anhydrate/anhydrous: When a molecule doesn’t have water molecules present in it. Anhydrous compounds may simply be compounds that have been dried, but are more commonly molecules with the capacity to loosely combine with water molecules on a microscopic level, but have not done so.
atomic mass unit (amu): Another term for an atomic mass unit is the Dalton (Da). Typically, most chemists don’t worry about either all that much, because we deal with macroscopic quantities such as “grams.”
atomic radius: The atomic radius of an atom is surprisingly hard to find because atoms have no theoretical end to how big they are. Instead, the electron density goes down to zero, but only at an infinite distance from the atom. As a result, the usual way of measuring atomic radius is to bond two atoms of the same element and declare the halfway point as the radius of the atom (this is referred to as the Van der Waals radius, not that you’ll ever need to know that).
aufbau principle: The word “aufbau” isn’t the name of a scientist, but instead comes from the German aufbauprinzip which means “building up principle.” Cool, eh?
Avogadro’s law: It’s important to keep in mind, however, that gases don’t actually behave like this in the real world. Though ideal gases are assumed to be infinitely small and have no intermolecular forces, this is not true for gases in the real world. However, under conditions you’re likely to bump into, this rule is usually within 5% or so, which is good enough for us.
base: However, there are other definitions. The Brønsted-Lowry definition of a base states that bases absorb the H+ ions that are given off by acids, and the Lewis definition states that bases are electron-pair donators. This means that the Arrhenius definition given above is correct, but incomplete. Interestingly, it also means that compounds are neither inherently Brønsted-Lowry nor Lewis bases, because they may function as either depending on what they’re reacting with. (Though, in practice you probably won’t see hydrochloric acid behave as a base by any definition).
beta particle (β): On the other hand, “beta-plus decay” (which I didn’t discuss above) is a type of radioactive decay where a positron is given off by a nucleus, forming an atom with an unchanged mass and decreasing the atomic number by one. Positrons are the antimatter version of electrons, so they have the same mass (essentially nothing) and a positive charge instead of a negative charge.
binding energy: Because Einstein told us that E = mc², the bound nucleus has more mass than it does when it breaks apart. Where does the mass go? It is turned into energy – lots of it – which is why nuclear reactions give off so much heat. This seems to violate the law of conservation of mass, so it’s more correct to say that the first law of thermodynamics is the “law of conservation of mass-energy” (though it’s rarely stated like that).
bond dissociation energy / bond energy: To visualize the difference between them, imagine the example of water. The bond dissociation energy is the strength of one mole of O-H bonds in water (i.e. the amount of energy needed to break that bond, usually in kJ/mol), whereas the bond energy is the amount of energy it takes to break all of the O-H bonds in one mole of water. These two values are the same for diatomic molecules, but not for anything else.
buffer: A buffer works by converting the weak acid to its corresponding conjugate base when a base is added to it, or the base to its conjugate weak acid if an acid is added. Though the pH of the solution will change when acid or base is added, this process essentially turns a strong acid into a weak one, or a strong base into a weak one, resulting in a much smaller pH change.
calorimetry: Calorimetry is frequently carried out in a device called a bomb calorimeter, which is a sealed container that contains whatever process is being studied. In bomb calorimetry, the heat generated by the process causes a water bath outside of the calorimeter to heat up, and this energy gain is equal to the amount of energy given off by the process. Incidentally, the “bomb” in bomb calorimeter refers to the fact that there is frequently a great deal of heat and pressure generated in it during the process that’s being studied, though it’s usually hoped that it doesn’t explode.
catalyst: Chemical reaction rates are dependent on the activation energy of the process, so when a catalyst comes up with a different reaction pathway that has a lower activation energy, the reaction speeds up. This is in contrast to an inhibitor or poison, which slows reactions by forcing the compounds to undergo reaction via a pathway with a higher activation energy.
chemical equation: Though I’ve heard a little bit of feedback that comparing equations to recipes is a little simplistic, I think it’s extremely appropriate to do so. Equations tell you what ingredients are needed to form some quantity of products. The symbols of state that are frequently shown give you tips about what equipment should be used and how you should work with the products. And symbols around the arrow tell you the specifics of the things you need to do to make the reaction take place. Though it’s true that scientific journals use written out procedures to describe chemical processes, this is generally because the reactions are fairly complex and require several isolation and purification steps.
chromatography: This is, of course, a huge oversimplification. Though I’ve described chromatography with a liquid mobile phase and a solid stationary phase, it’s not at all uncommon to have a gaseous mobile phase (as in gas chromatography) or a gel stationary phase (as in gel electrophoresis). Additionally, there are a wide variety of ways that the compounds can stick to the stationary phase, using everything from polarity, to electrical charge, to size. Though chromatography is sometimes described as a process used for separation, the main use of chromatography is in the identification of chemical compounds, as it’s simply too much of a pain in the butt to separate large quantities of a compound through the process.
colligative property: There has been some discussion about whether taste and color are colligative properties. My take on this is that the basic taste of something is not a colligative property (the same taste will be present regardless of concentration), but the intensity of the taste will be a colligative property. Same deal with color.
condensation: The reason condensation occurs in both of these examples is that the vapor comes into contact with something colder than the boiling point of the compound, causing it to instantly condense. You can see cases where the vapor condenses through gradual cooling when something is distilled, or when water collects at the lid on a boiling pot.
conjugate acid/base: Many compounds can act as both acids and bases, depending on what they react with. Water, for example, is a compound that we usually think of as neutral. However, water acts as a base when reacting with hydrochloric acid, and as an acid when reacting with ammonia. As a result, it’s not technically correct to say that [some compound] is a Brønsted-Lowry acid, but rather that it acts like a Brønsted-Lowry acid in some particular example.
continuous spectrum: A continuous spectrum is generated by a different process than a line spectrum. If you add energy to some element, the chances are excellent that the electrons will absorb the energy, rise to an excited state, and then give off one color of light when returning to the ground state – this causes a line spectrum that’s unique to this element. A continuous spectrum, on the other hand, is caused by blackbody radiation, in which thermal energy is emitted over the entire electromagnetic spectrum. This is an extraordinarily difficult process to explain, so I will leave that for people smarter than me.
covalent bond: Interestingly, it’s possible to have quadruple bonds, quintuple bonds, and sextuple bonds, though they’re all extraordinarily rare and occur only in transition metals (with the exception of a C-C quadruple bond observed in dicarbon).
critical mass: How you achieve a critical mass is important, depending on what you plan on achieving. One way of doing it is to compress a subcritical amount of radioactive material into a far more dense solid, causing the atoms to be closer together and the decay processes to result in a higher quantity of nuclear reactions – this is the process that takes place in an implosion-type nuclear bomb. In nuclear reactors, a critical mass is generally achieved by putting rods containing nuclear material into close proximity with one another – this results in a very easily-controlled critical mass. (It’s also possible to make a nuclear weapon using this method (called a “gun-type fission weapon“), but it was only used in very early American and South African nuclear weapons.
Dalton’s law of partial pressure: Though usually phrased in the format above, it’s usually not how Dalton’s law is actually used. Instead, it’s the idea of Dalton’s law that’s more important: namely, that each gas in a mixture of gases can be treated independently of the others. As a result, if you want to find the pressure of a mix of gases, you can simply use PV = nRT with the overall number of moles of all gases combined, because Dalton’s law tells us that their identity doesn’t really matter. Of course, this goes back to the concept of an ideal gas, which Dalton didn’t come up with, but Dalton’s law really is the most dramatic way that first-year chemistry students are likely to see this concept.
decomposition: A general equation for this process is A -> B + C, which denotes that compound A has been turned into two other compounds. One example of this occurs during the electrolysis of water in which liquid water is converted into hydrogen gas and oxygen gas.
dehydration: When water is removed from something. This can occur either in the sense of the removal of water from hydrates, or in the sense of removal of a water molecule due to some chemical reaction.
delocalization: An also in many, many, many other cases. Aromatic molecules are only one class of compound with electron delocalization. In fact, molecular orbital theory makes the assumption that all of the electrons in a compound are somehow shared among them all, resulting in the formation of molecular orbitals. I wouldn’t try explaining that to your teacher, though.
denature: Basically, proteins (including enzymes) have three-dimensional structure (secondary, tertiary, and quaternary structures) that allow them to perform certain functions. When denatured, these structures are altered such that the functions can no longer be performed. Given that enzymes are needed for the body to work, their denaturing by conditions such as acidosis is a Very Bad Thing.
distillation: There are many, many different types of distillation, but they all basically have this main idea in common. Though the student will probably see distillation as a small-scale operation that takes a long time to perform, large-scale distillations are performed in a large number of industries. Link.
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The Cavalcade o' Chemistry 