Chemistry Vocabulary (M-Q)

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 M-Q, 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.


main-block elements:  Elements in groups 1-2 and 13-18 of the periodic table.  The chemistry of these elements depends largely on the interactions of s- and p-orbitals, thouh this is less true for elements with higher atomic masses.*

mass defect:  The difference between the mass of the nucleus of an atom and the mass of the nucleons (protons and neutrons).  Typically, the mass of a nucleus will be less than that of the constituent nucleons because some of the mass that’s present in the nucleons has been converted to the energy used to hold them together (the “binding energy.”)*

mass:  The amount of matter in an object.  The more mass, the more stuff is present. Mass and weight are very closely related ideas, and the terms have little practical difference if you live on Earth.*

mechanism:  A step-by-step sequence that shows how the products of a reaction are made from the reagents.  The overall rate of a chemical reaction is determined by the rate of the slowest step (referred to as the “rate-determining step.“)

molality (m):  The number of moles of solute dissolved per kilogram of solvent in a solution. For example, a solution in which 2 moles of sodium chloride are dissolved in 1 kg of water has a concentration of 2 m.*

molar mass:  The mass of one mole of particles.  These particles can be molecules, atoms, or anything else.*

molar volume: The volume of one mole of a gas at standard temperature and pressure (0 degrees Celsius, 100 kPa). Because all gases are usually treated as ideal gases (which isn’t exactly right, but close enough), the molar volume of gases is 22.4 liters.

molarity (M):  Molarity is a unit of concentration equal to moles of solute dissolved per liter of solution.  Take care not to mix this up with molality (m), because you’ll look silly.*

mole fraction (χ):  The mole fraction of some component in a solution is equal to the number of moles of that component divided by the sum of the number of moles of all components.  For example, if you had a solution in which 4.0 moles of ethanol were mixed with 8.0 moles of water, the mole fraction of ethanol would be 4.0 / (4.0 + 8.0) = 0.33. Mole fraction is a unitless number, and the mole fractions of all components in a mixture will add up to 1.

mole ratio:  The mole ratio is a term used in stoichiometry to describe the relationship between the number of moles of what you’re given to the number of moles of what you’re trying to find.  The coefficients in the mole ratio are obtained from the balanced chemical equation for the reaction, with both values being the same.

mole (mol): 6.02 x 10²³ things.  You can think of the term “mole” as being like the term “dozen“, “pair“, or “gross“, as each term represents some number of objects.  The term “mole” is generally used with atomic-sized objects such as ions, atoms, and molecules because one mole of anything you can see would weigh a lot and probably wouldn’t be practical.*

molecular compound:  Another word for “covalent compound”, as covalent compounds are the only ones that form molecules.

molecular formula: A formula that shows the correct number of all of the atoms in a molecule.  For example, water has a molecular formula of H2O, indicating that there are two atoms of hydrogen and one atom of oxygen in a single molecule.  Ionic compounds are sometimes said to have molecular formulas to represent the ratios of the elements present, but this is more properly referred to as a chemical formula.

molecule:  A bunch of atoms that is bound to each other with covalent bonds and has no charge.

monatomic ion:  An ion that contains only one atom.  Examples include Na+, Cl-, and Fe+2.

neutralization reaction:  A reaction in which an acid and a base combine to form water and a salt (ionic compound).  These reactions are used in titrations to determine the concentration of an acid or base.  Also called an acid-base reaction.

node:  A location in an orbital in which there is no electron density.  One example of this is the nucleus in a 2p-orbital, which has a node at the nucleus.

nonpolar covalent bond:  A covalent bond in which electrons are shared equally between the two atoms.  This can be determined using electronegativity values – if the differences between the two elements is less than 0.4, the bond is said to be nonpolar.*

normal boiling point:  The boiling point of a substance at a pressure of 1 atm.*

normal melting point:  The melting point of a substance at a pressure of 1 atm.*

normality (N):  The number of equivalents of a substance dissolved in a liter of solution. An equivalent refers to the number of atoms in the compound that are involved in the reaction, which leads to interesting cases where solutions with identical molarities may have different normalities, depending on what they’re reacting with.

nuclear fission:  When a large atom breaks apart to form smaller ones.  This very rarely occurs as a radioactive decay process, but is most often observed through artificial bombardment of radioactive elements with neutrons (this occurs in nuclear reactors and atomic bombs).  Though nuclear fission gives off a great deal more energy than chemical processes, it gives off much less energy than fusion processes.

nuclear fusion:  When small atoms combine to form another one.  The main types are thermonuclear fusion (the atoms are heated to millions of Kelvin) and magnetic or inertial confinement fusion (a target is hit with a great deal of energy to initiate the process).*

nuclear reaction:  Any process in which the nucleus of an atom is changed in some fashion. This includes fusion, fission, and radioactive decay.  Nuclear reactions require a huge amount of energy to get started, which is why you don’t see them happen much.*

nucleon:  Protons and neutrons, as both are present in the nucleus.

nucleus:  The middle part of an atom where the protons and neutrons are found.*

octet rule:  “All atoms want to be like the nearest noble gas.”  Or, to flesh it out a little bit, atoms will tend to gain or lose electrons to gain the same electron configuration as the closest noble gas.  This rule explains how electrons are transferred in ionic compounds, why they’re shared in covalent compounds, and ideas such as chemical reactivity and electronegativity.  It’s not really an exaggeration to say that if you really understand the octet rule, you probably understand chemistry really well.*

optical isomerism (chirality): Isomerism in which the isomers cause plane-polarized light to rotate in different directions.  In organic molecules, this most often happens when a carbon atom (called a stereocenter or stereogenic center) has four different substituents. Chiral molecules react differently with the various optical isomers of other molecules, which explains why we often see one isomer of a medication treat illness while the other doesn’t do anything at all.*

orbital:  Where the electrons live in an atom.  Orbitals are not places where electrons orbit the nucleus and they’re also not places where electrons jump from place to place (despite the fact that many chemistry teachers explain it in this way).  Instead, orbitals are potential areas that have the shape and energy of the electrons that may occupy them.  In other words, when electrons are in orbitals, they are not points (as you normally think of them), but three-dimensional standing waves.*

organic compound:  A chemical compound that contains carbon, with the exception of carbides, oxides, carbonates, and cyanides.

osmosis: The flow of a liquid into an area of higher concentration through a semi-permeable membrane. This will occur until the concentration of the two liquids is identical, or until the osmotic pressure is greater than the force of the solvent trying to enter the solution.*

oxidation number: The apparent charge on an atom.  For example, the Fe+2 ion has an oxidation number of +2.  There’s more to it than that, but it’s a little too complicated for a vocabulary list.

oxidation: When a substance loses electrons to give it a more positive charge.  For example, if a reaction involves the conversion of copper (Cu) to the copper(I) ion (Cu+), this would be an oxidation process.  If one compound is oxidized, another compound must have been reduced (gained electrons) because the electrons had to go somewhere.

oxidizing agent: A compound that causes another element to be oxidized by taking electrons away from it.  As a result, they are reduced.

partial pressure:  If you have a mixture of gases, each of them is seen as having their own contribution to the overall pressure of a gas – this value is the partial pressure of that gas. For example, our air contains about 21% oxygen, which means that the partial pressure of oxygen in our atmosphere is 0.21 atm.*

Pauli exclusion principle:  No two electrons in an atom can have the same quantum numbers.  Or, put another way, no two electrons in an atom can be identical.

Pauling electronegativity scale:  The electronegativity of an element is typically given in Pauling electronegativity units, which range from 0.7 (francium) to 3.98 (fluorine).  This scale is used to, among other things, determine bond character and reactivity.*

percent yield: A method of figuring out the success of a chemical reaction, measured by dividing the actual yield (i.e. the amount of stuff you actually collect after the reaction is over) by the theoretical yield (i.e. the amount of stuff that stoichiometry calculations predicted you’d make before you get started.)  Turn that into a percent, and you’ve got yourself the percent yield!  Example:  If stoichiometry predicted you’d make 25 grams of product and you only made 15 grams, the percent yield would be (15 g/25 g) x 100% = 60.%

period: A row (from left to right) in the periodic table.  Elements sharing a period have very little in common with one another, as it’s the number of valence electrons and not the overall orbital energy that really affects chemical bonding.  Also, a period is at the end of this sentence.

periodic law: As you look at elements by increasing atomic number, you find that the properties of the elements seem to periodically repeat themselves (i.e. element 9 has similar properties to element 17 – both are halogens).  We now know that this is because elements in the same group have similar electron configurations, but back in the old days they thought magic spiders were responsible for this effect.*

periodic trend:  Any characteristic of elements that gradually varies as you move across a period or down a group in the periodic table.  These include electron affinity, electronegativity, atomic radius, and ionization energy.  Trends that change across a period are largely caused by the octet rule, while those that change down a group are dominated by the shielding effect – the exception for both is atomic radius.

pH: The primary scale used for determining the acidity of a solution.  If a solution has a pH less than 7, it is acidic.  If the pH is exactly 7, it is neutral.  If the pH is greater than 7, it is basic.*

phase diagram:  A chart that indicates the state of matter of a material under different conditions of temperature and pressure.  Interesting (?) features on a phase diagram include the triple point (all three phases in equilibrium), lines between each phase (where both phase are stable), and the critical point (past which the material turns “supercritical“, meaning that the distinction between gas and liquid vanishes).*

phase:  The state of a compound (solid, liquid, gas, plasma, etc.)*

physical change:  Any change that doesn’t cause something to become a different material, from a chemical standpoint.  Hitting my neighbor with a brick is a physical change (he is still the same annoying neighbor), while setting his house on fire is a chemical change (the house turns into stuff that’s no longer wood).  Not that I’d do that to my neighbor.

physical property: A property of a material that doesn’t describe how it turns (or potentially turns) into another material.  For example, melting point is a physical property because it describes how a material turns from solid to liquid while remaining the same thing, while flammability is a chemical property because if something burns, it’s a whole new material.

pi-bond (π-bond):  The second or third bond between two atoms, caused by the overlap of unhybridized p-orbitals on each.

pKa: The most commonly-used form of the acid dissociation constant, pKa is a measure of the relative strength of an acidic proton.  Acids with a pKvalue less than about -2 are considered strong acids, while others are weak acids.  Generally speaking, the more positive the pKa of a material, the less acidic it is.

polar covalent bond:  A covalent bond in which both atoms have dissimilar electronegativities (a difference in electronegativity between 0.4 and 1.7 Pauling units), indicating an unequal sharing of electrons.*

polyatomic:  Having more than one atom.  Usually refers to polyatomic ions such as OH-.

polymer:  A molecule containing many repeating units (monomers).  Plastics are a very common example of polymers, as are proteins and DNA (interesting link).

polyprotic acid:  An acid that contains more than one proton (i.e. more than one acidic hydrogen).  Examples include phosphoric acid (H3PO4) and sulfuric acid (H2SO4).  This is in contrast to monoprotic acids such as hydrochloric acid (HCl).

potential energy: Stored energy. In chemistry, potential energy usually takes on the form of chemical bonds.  According to the law of conservation of energy, energy can be converted from potential to kinetic energy and back again.

precision:  A measurement of how repeatable a measurement is – this is manifested by a higher number of significant figures, indicating that the tool used to take the measurement gave a great deal of meaningful data.  Precision is distinct from accuracy, which is a measurement’s closeness to the actual value of something (though the two are related, sort of).*

pressure:  A measure of how much push something has exerted against it.  To be a little less vague, it’s measured as the force applied to an object divided by the area over which it has been applied.*

product:  The compound formed in a chemical reaction.

proton: It is the particle with positive charge in the nucleus of an atom.  An alternate use for the word proton is used to describe the hydronium ion (H+) in acid-base chemistry.

quantum theory:  A theory that describes the behavior of electrons in an atom, as well as a bunch of other stuff we usually don’t worry about in introductory chemistry.  Key ideas include the quantization of energy (i.e. electrons can only have particular quantities of energy when in an atom), the nature of electrons in atoms as being 3-D structures rather than points, and some weird stuff about causality and whatnot.*


main-block elements:  Some scientists believe that group 12 (Zn through Cn) should be main-block elements due to their properties, but it’s not gonna happen.

mass defect:  It’s this energy that’s given off during a nuclear reaction.  And it’s a lot of energy, which you’d expect given that it’s able to keep protons together in the nucleus despite the fact they both have positive charge.

mass:  If you’re reading this, you’re probably a physics guy who wants to kill me.  Yes, I’m well aware that mass and weight are not the same thing, and that weight is related to both mass and the force of gravity exerted on the object.  However, you’ve got to admit that for people on the surface of the earth, they’re the same thing.  And since nobody reading this is very likely to ever leave the surface of the earth (by either going to space or to the center of the earth), they don’t need to worry about it much.

molality (m):  For aqueous solutions, the kilograms of water are the same as the number of liters of water used, because one liter of water weighs one kilogram.  However, be careful to make sure that you are using kilograms of solvent rather than kilograms of solution when calculating molality.

molar mass:  There are lots of terms related to molar mass that you need to be aware of:

  • molecular weight and formula weight:  Equal to the molar mass of a compound divided by the molar mass constant.  In other words, it’s the molar mass with the unit dropped from the end.  It’s not uncommon to see these terms used interchangeably with molar mass.
  • molecular mass:  The mass of a single molecule of a substance.  Because we usually talk about moles, the molar mass is equal to the molecular mass times Avogadro’s number (6.02 x 10^(23)).  Again, this term is sometimes used interchangeably with molar mass.
  • gram formula mass:  Equal to the molar mass of a compound.  This term is fairly common in high school chemistry classes, but not anyplace else.

molarity (M):  A common mistake that students make is to assume that they can tell the volume of the solution before they actually make it.  For example, if you mix a solute with a volume of 10.0 mL with 950.0 mL of water, you might assume that the resulting solution has a volume of 960.0 mL.  In fact, this is not the case, and the volume of the solution would be less than 960.0 because the solute particles will tend to fit between the solvent particles, resulting in a denser liquid (link).

mole (mol): First off, I have no idea why the “abbreviation” for mole is “mol” – you’d think that an actual abbreviation would actually significantly shorten the word.  Secondly, addressing the concept of how big a mole is, let’s imagine having a mole of severed human heads (frighteningly enough, somebody measured this and found that the average mass of a human head is about 5 kg and somebody else found that the volume of a human head is about 3 L. Given these numbers, the mass of a mole of human heads would be 3.0 x 10^(27) grams and the volume of a mole of human heads would be 1.8 x 10^(24) liters.  In other words, the mass of a mole of human heads would be about half of the mass of the planet Earth, and the volume of these heads would be about 1.4 times the volume of our planet.  By comparison, one mole of water has a mass of 18 grams (a little more than the mass of a single AA battery) and a volume of 18 mL (a little more than the volume of 2 AA batteries).  Even ignoring ethical considerations, it’s still inappropriate to use the unit “moles” for large objects.

nonpolar covalent bonds:  In a very strict sense, any bond in which the two atoms are not in completely different chemical environments will be polar to some extent.  However, bonds with a difference less than 0.4 units are pretty darned nonpolar, so that’s considered close enough.  Please note that elements with a difference of 0.3 units don’t bond significantly different than atoms with a 0.4 unit difference, as the distinction has been made fairly arbitrarily.  Likewise, the usual cutoff of 1.7 Pauling electronegativity units doesn’t instantly cause a transfer of electrons – even compounds like NaF have some sharing of electrons (though in that case, not much).

normal melting/boiling point:  This is in contrast to the standard melting/boiling point of a liquid, which is the temperature at which the liquid melts/boils at a pressure of 1 bar (0.99 atm).  Given the small difference, it’s not really something most people worry about unless they’re doing very high precision work.

nuclear fusion:  Thermonuclear fusion is the process that takes place in nuclear weapons, in which the “primary” (a fission explosion) initiates the “secondary” (the fission reaction).  At this point, fusion power plants are not used because of technical difficulties, but it has been claimed by Lockheed/Martin that they will have a prototype of a working reactor by 2017 with a full-scale model by 2022.  Yeah, right.

nuclear reaction:  When chemists use nuclear reactions, they generally use existing processes such as radioactive decay rather than trying to start and control nuclear reactions on their own.  Nuclear processes simply take too much energy to initiate in anything but the most kick-ass of labs, so they’re not terribly handy in synthetic processes.

nucleus:  We don’t really understand how the nucleus works, though we’re trying.  It’s clear that the nucleus has different energy states, but the reasons for this are not well-understood.  As a result, there are a lot of weird guesses about how nuclei will behave, but not much that we can agree on to support it.

octet rule:  I had a friend in grad school say that he was studying his first year chemistry books to study for his Ph.D. defense because it was his opinion that if you really understood that stuff, you really understood chemistry.  He was a super-smart guy and he’s currently got his Ph.D., so I guess there was something to it.

optical isomer:  Several points worth mentioning:

  • When I was learning organic chemistry, chiral centers were called “stereocenters.”  The accepted term is now “stereogenic center”, though both are sometimes used.
  • The acid-reflux medicine Nexium contains only the active isomer present in Prilosec (which contains both isomers, one of which is inactive).
  • There are several different ways of distinguishing stereoisomers from each other.  R/S naming is based on an ordering of the substituents, while (+)/(-) and d/l notations refer to the direction in which polarized light rotates, and D-/L- notation compares the structure of the molecule to glyceraldehyde.  This can sometimes cause confusion in new chemists, because d/l and D-/L- notations are not related to each other in any way.
  • d/l notation comes from the terms “dextrorotatory” (it moves light in a clockwise/right direction) and “levorotatory” (it moves light in a counter-clockwise/left direction). R/S naming refers to the Latin for right and left, where R stands for “rectus” (right) and S stands for “sinister” (left).  However, R/S notation doesn’t refer to the direction that polarized light rotates, but rather the priority assigned to each of the constituents on the isomer.  (Random note:  Left-handers were historically thought of as somehow evil because of their “sinister” way of doing things.  My son is left-handed, and I am inclined to agree).

orbital:  Please, please, please don’t believe that electrons jump around inside of orbitals (THIS is WRONG WRONG WRONG!)  They don’t – they’re three-dimensional waves that are described by mathematical functions called wavefunctions.  To understand this, consider the function x²+y²=1, which describes a circle with a radius of 1.  If this were the wavefunction of an electron, the electron would not be a point that jumped around on the circle, but rather something that occupies the circle itself.

osmosis: Osmosis is something that’s unbelievably important in biological systems.  For example, living cells are kept “inflated” by the fact that they have a higher concentration of solute inside of them than outside, which keeps the solvent pouring into it.  If you have osmosis taking place, eventually the pressure inside of the cell will build up to the point that the solvent can’t enter the cell anymore – this is called the “osmotic pressure.”

partial pressure:  When working with gases, we make the assumption that each gas is invisible to the others – this makes calculations simpler because we can solve problems for each gas individually without worrying about how the other gases affect things.  This assumes that gases are all ideal – not true, but close enough for us.

Pauling electronegativity scale:  Though the Pauling scale is the one most-commonly used (and pretty much the only one used in introductory chemistry classes), there are other scales that use different standards to determine electronegativity.  Not surprisingly, they all tend to show the same trends.

periodic law: It’s not often mentioned, but magic spiders were seen as the cause of most unexplained phenomena back in the 18th century.  And when it wasn’t magic spiders, they usually just said that “demons did it”, which explains why Karl Wilhelm Scheele was killed and eaten by paranoid villagers in 1786.

pH:  The term pH itself is a little confusing, because nobody’s sure whether it stands for “potential of hydrogen” or “power of hydrogen.”  Not that it matters.  Anyhow, though a pH of exactly 7 is treated as neutral while one of 6.99999 would be acidic, we usually think of solutions with a pH somewhere around 7 as being “neutral” in our everyday lives, as solutions with a pH from about 6-8 don’t really bother us much.  I wouldn’t try that on a chemistry test, though.

phase diagram:  It turns out that phase diagrams can be more complicated than this in some cases.  For example, it’s possible to make a phase diagram for water in which not only the solid, liquid, and gas are present, but also the eleven different crystal structures into which the solid can arrange itself (link).  The most famous of these structures aside from our familiar ice I is ice IX, though for this reason and not this reason.

phase:  It’s annoying to actually name the phases because people seem to disagree about them.  Solid, liquid, and gas are all well-agreed on, but some people define plasmas as a gas, and others insist on including Bose-Einstein condensates despite the fact that they wouldn’t know one if it bit them on the butt.  If you go with the four I mentioned above, however, you’ll be fine.

pKa: To give you an idea of exactly how un-strong an acid with a high pKa is, let’s use the example of methane.  Methane has a pKa somewhere between 48 and 60 – exactly which is unclear – which means that if you turned the planet Mars into methane and dissolved it in water, only one of those molecules would lose an H+ ion (assuming you used the value of 48 for pKa).  Using the upper limit of 60, the amount of hydrogen you’d need would have the same mass as the Segue 2 dwarf galaxy.

polar covalent bond:  I mentioned this when speaking of nonpolar covalent bonds, but thought it was worth mentioning again:  The distinction between nonpolar covalent, polar covalent, and ionic bonding is made for the convenience of chemists rather than because there’s any magical cutoffs between them.  It’s true that atoms with similar electronegativities act as we’d normally expect “nonpolar covalently bonded” atoms to behave (likewise for polar covalent and ionic bonding, respectively), but the cutoff of 0.4 Pauling units between nonpolar and polar covalent, as well as the cutoff of 1.7 Pauling units for covalent and ionic are pretty much just guidelines.  In reality, there’s almost no difference in bond character between bonds with differences of 0.3 and 0.4 units, and no difference between bonds with differences between 1.6 and 1.7 units.  Polarity is not something with discrete categories, but rather a continuum.

precision:  Textbooks will always tell you that precision and accuracy are totally different phenomena, and from a literal standpoint, that’s true.  For example, if you write down a bunch of digits in a measurement, that doesn’t mean they’re actually right.  However, consider this:  Who in their right minds would design an instrument that gives values to a large number of decimal places if the measurement wasn’t accurate?  For this reason, it can usually be inferred that high precision = high accuracy, but various problems (dishonest instrument manufacturers, lack of calibration) may cause this to be untrue.

pressure:  To give you an idea of the difference between pressure and force, consider getting poked in the butt with a needle.  Though somebody jabbing you with a pen will probably exert more force onto your butt than somebody jabbing you with a needle, the needle will be the one to pierce your skin because the area over which this force is applied (i.e. the area of the point of the needle) is much smaller.  This brings the pressure way up, causing it to poke into you.  Incidentally, don’t let people poke you in the butt with sharp things.

quantum theory:  To demonstrate these points (and many other things that are hard to understand), physical chemists and physicists use a dazzling variety of equations – the one that we use for electrons is called the “wavefunction.”  I took two classes in quantum chemistry in college and grad school, and let me tell you that this stuff is complicated!  If your teacher claims that he or she understands quantum mechanics, consider the words of Nobel Laureate Richard Feynman who said that “If you think you understand quantum mechanics, you don’t understand quantum mechanics” (link).  And this from the guy who invented quantum electrodynamics.

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