1,800+ MCAT terms, defined.

Halogens are the Group 17 (VIIA) elements: F, Cl, Br, I and At. Each has seven valence electrons, so it readily gains one electron to form a halide ion, X⁻. Most testable: going down the group, electronegativity and oxidizing strength de...

A reaction rate is the change in concentration of a reactant or product per unit time (mol/L·s). The most testable point: the rate law, rate = k[A]^m[B]^n, has orders found from experimental data, not from the balanced equation's coeffic...

An electron configuration lists the subshells an atom's electrons occupy and the number in each, such as 1s² 2s² 2p⁴ for oxygen. Electrons fill by Aufbau order, Pauli exclusion and Hund's rule; transition-metal cations lose ns electrons...

Molality (m) is moles of solute per kilogram of solvent. Because it uses mass rather than volume, it does not change with temperature, so colligative property equations such as ΔTf = i Kf m use it instead of molarity.

Oxidation state (oxidation number) is the hypothetical charge an atom would have if every bond's shared electrons went to the more electronegative atom. Oxidation is a rise in oxidation state and reduction is a fall, so it shows what is...

Transition metals are the d-block elements (groups 3–12), whose atoms or common ions typically have a partially filled d subshell. They usually show several oxidation states, form colored complex ions, and lose their ns electrons before...

Atomic radius is the size of an atom, usually estimated as half the distance between the nuclei of two identical bonded atoms. It typically decreases across a period (higher effective nuclear charge) and increases down a group (added ele...

Dilution means adding solvent to a solution so the solute concentration falls while the moles of solute stay the same. The key relationship is M₁V₁ = M₂V₂, so adding solvent lowers molarity in inverse proportion to the volume increase.

Average kinetic energy is the mean translational energy of the particles in a sample, equal to (3/2)kBT for an ideal gas. It is proportional to absolute temperature in kelvins and does not depend on the gas's identity or mass, so at the...

Hund's rule says electrons fill degenerate orbitals (such as the three 2p orbitals) one at a time with parallel spins before any orbital gets a second electron. The most testable point is that it sets the number of unpaired electrons, wh...

Two quantities are proportional when their ratio is constant, so y = kx: doubling x doubles y, and the graph of y versus x is a straight line through the origin. Most testable: gas-law temperature must be in kelvin, because 0 °C is not a...

A net ionic equation shows only the species that actually change in a solution reaction, after removing spectator ions that appear unchanged on both sides. Aqueous strong electrolytes are written as ions; solids, gases, and weak electrol...

Partial pressure is the pressure one gas in a mixture would exert if it alone filled the container at the same temperature. For ideal gases it equals the gas's mole fraction times the total pressure, and the partial pressures add up to t...

The density of water is its mass per unit volume, about 1.00 g/mL or 1.00 × 10³ kg/m³ near room temperature. The most testable point is that ice is less dense than liquid water, because hydrogen bonds hold it in an open lattice, so ice f...

A buffer solution contains a weak acid and its conjugate base (or a weak base and its conjugate acid) and resists large pH changes when small amounts of strong acid or base are added. Its pH depends on the pKa and the base-to-acid ratio,...

Atomic weight is the average mass of an element's atoms as found in nature, weighted by isotope abundance, in atomic mass units (u). It is the periodic table value and equals molar mass in g/mol. Testable point: it is a weighted average,...

Bond order is the number of electron pairs shared between two bonded atoms: 1 for single, 2 for double, 3 for triple bonds. Between the same two atoms, a higher bond order generally means a shorter, stronger bond. Resonance and MO theory...

An Arrhenius acid raises the H⁺ (H₃O⁺) concentration in water; an Arrhenius base raises the OH⁻ concentration in water. It is the narrowest acid-base definition, so NH₃ is a Brønsted-Lowry base but not an Arrhenius one. The name also tag...

Reactants are the substances on the left of the arrow in a chemical equation. They are consumed as bonds break and atoms rearrange into products. Most testable: the limiting reactant, the one that runs out first, sets the maximum amount...

Electronic geometry is the arrangement of all electron domains (bonding groups and lone pairs) around a central atom, predicted by VSEPR. It counts lone pairs, unlike molecular geometry, which describes only the positions of the atoms.

Paramagnetic describes an atom, ion, or molecule with one or more unpaired electrons, which is weakly attracted into an external magnetic field. Diamagnetic species have their electrons paired and are weakly repelled. To classify one, wr...

A nonmetal is an element on the upper right of the periodic table (plus hydrogen) with high electronegativity and ionization energy. Nonmetals tend to gain electrons to form anions or share electrons in covalent bonds. Metal + nonmetal u...

Orbital hybridization is a valence bond model in which an atom's s and p orbitals mix into equivalent hybrid orbitals, one per electron domain (bonded atom or lone pair). Two, three or four domains give sp, sp² or sp³, which sets geometr...

Bond dissociation energy is the enthalpy needed to break one mole of a specific bond in the gas phase, with each atom keeping one electron. It is positive. Stronger, shorter, higher-order bonds generally have larger values, and ΔH ≈ Σ BD...

A phase diagram plots pressure against temperature and shows which phase of a substance (solid, liquid or gas) is stable in each region. Lines are two-phase equilibria and the triple point has all three. Testable: the solid–liquid line t...

Noble gases are the Group 18 elements (He through Rn). Their filled valence shells give them the highest ionization energies in their periods and make them generally unreactive, though Xe forms compounds such as XeF₄. Boiling points rise...

Gay-Lussac's law says that for a fixed amount of gas at constant volume, pressure is directly proportional to absolute temperature: P/T = constant. Temperature must be in kelvin, so doubling T in kelvin doubles P.

Reaction order is the exponent on a reactant's concentration in the experimentally determined rate law. The overall order is the sum of the exponents. Most testable: find the order from initial-rate data by seeing how the rate changes wh...

Fusion is melting, the solid-to-liquid phase change, which absorbs heat (ΔHfus > 0) at a constant temperature. The testable point is that heat added at the melting point breaks intermolecular attractions instead of raising temperature. (...

A subshell is the set of orbitals within a shell that share the same n and l, labeled s, p, d or f (such as 2p or 3d). A full s, p, d or f subshell holds 2, 6, 10 or 14 electrons, since it has 2l + 1 orbitals with two electrons each.

The transition state is the highest-energy, unstable arrangement of atoms along a reaction pathway, with bonds partly broken and partly formed. Its energy above the reactants is the activation energy, and a catalyst speeds a reaction by...

Electromotive force (emf) is the potential difference, in volts, that a galvanic cell can supply between its electrodes; despite the name, it is not a force. E°cell = E°cathode − E°anode, and a positive value means the cell reaction is s...

Boyle's law says that for a fixed amount of gas at constant temperature, pressure and volume are inversely proportional, so PV stays constant. Halving the volume doubles the pressure. It is an ideal gas result that real gases follow clos...

A Lewis structure draws a molecule or ion's valence electrons as bonds (shared pairs) and lone pairs. Most testable: the best structure or resonance contributor usually has formal charges near zero, with any negative charge on the more e...

London dispersion forces are weak attractions between atoms or molecules, caused by temporary dipoles that induce dipoles in neighbors. They act in every substance and grow with polarizability (more electrons, larger size), so for simila...

An acceptor is the species that receives something in a reaction: a proton (Brønsted-Lowry base), an electron pair (Lewis acid), or electrons (oxidizing agent). Testable point: identify what is accepted, since a Lewis acid and a Brønsted...

A donor is the species that supplies something to a partner. Mainly it is an electron-pair donor (a Lewis base, such as a ligand) that forms a coordinate covalent bond with a Lewis acid. A Brønsted-Lowry acid is a proton donor, and in a...

A redox reaction transfers electrons: one species is oxidized (loses electrons, oxidation number rises) and another is reduced (gains electrons, oxidation number falls). The two occur together, and the oxidizing agent is the species that...

Trigonal bipyramidal is the VSEPR electron geometry for five electron domains around a central atom (sp³d), with 120° and 90° angles. Most testable: lone pairs take equatorial positions, giving seesaw, T-shaped, or linear molecular shapes.

Diamagnetic describes an atom, ion, or molecule whose electrons are all paired, so it has no net magnetic moment and is weakly repelled by an external magnetic field. Testable point: no unpaired electrons means diamagnetic; one or more m...

An element is a pure substance whose atoms all have the same atomic number Z (the same number of protons), and it cannot be broken into simpler substances by chemical means. The most testable point: identity is set by proton count, not b...

Kinetic molecular theory models an ideal gas as particles in random motion with negligible volume, no intermolecular forces, and elastic collisions. Its key result: average kinetic energy depends on temperature in kelvins alone, so at on...

A dipole-dipole force is the intermolecular attraction between the δ+ end of one polar molecule and the δ− end of another. For molecules of similar size, it is usually stronger than London dispersion alone, so polar compounds tend to hav...

A reaction mechanism is the series of elementary steps that add up to the overall reaction, including any intermediates. Key MCAT point: a proposed mechanism must sum to the balanced equation and agree with the experimental rate law, whi...

An adiabatic process is one in which no heat is exchanged between the system and its surroundings (q = 0). Energy can still change by work, so ΔU = −W. An ideal gas that expands adiabatically and does work cools, and one that is compress...

Charles's law says that for a fixed amount of gas at constant pressure, volume is directly proportional to absolute (Kelvin) temperature, so V₁/T₁ = V₂/T₂. The most testable point is that T must be in kelvins: doubling the Kelvin tempera...

A coordinate covalent bond is a covalent bond in which one atom, the donor with a lone pair, supplies both shared electrons to an acceptor with an empty orbital. The donor is the Lewis base and the acceptor is the Lewis acid. Once formed...

A chemical formula uses element symbols and subscripts to show which atoms a substance contains and in what ratio. The empirical formula is the simplest whole-number ratio; the molecular formula gives the actual atom counts and is a whol...

Gibbs free energy (G = H − TS) is the energy available to do useful work at constant temperature and pressure. A negative ΔG means a reaction is spontaneous as written, zero means equilibrium, and ΔG = ΔH − TΔS shows how temperature can...

A calorimeter is an insulated device that measures heat released or absorbed in a process by the temperature change of a known mass of water or other material. Most testable: heat lost by the system equals heat gained by the surroundings...