1,800+ MCAT terms, defined.

Van der Waals forces are weak attractions between molecules or nonbonded atoms. Broadly they include London dispersion and dipole-dipole forces; some texts mean dispersion alone. Testable point: dispersion forces strengthen with size and...

Fick's law says the net flux of a solute by diffusion is proportional to the concentration gradient and points from high to low concentration. Testable point: doubling the gradient doubles the flux, and a larger diffusion coefficient D (...

A coordinate covalent bond is a covalent bond in which both shared electrons come from one atom, the donor (a Lewis base), and go to an electron-poor acceptor (a Lewis acid). Once formed, it is identical to any other covalent bond of tha...

The empirical formula gives the simplest whole-number ratio of atoms of each element in a compound. A molecular formula is a whole-number multiple of it, so different compounds can share one (C₂H₂ and C₆H₆ are both CH). Finding it from m...

Faraday's constant (F) is the magnitude of the charge carried by one mole of electrons, about 96,500 C/mol (9.65 × 10⁴). It converts between coulombs and moles of electrons, so it links current and time to the amount of product in electr...

A conjugate acid is the species formed when a Brønsted-Lowry base accepts a proton, so it has one more H⁺ than the base (NH₃ → NH₄⁺). The stronger the base, the weaker its conjugate acid, and for a conjugate pair Ka × Kb = Kw.

The change in Gibbs free energy (ΔG) tells whether a process is thermodynamically favorable at constant temperature and pressure. ΔG < 0 is spontaneous (exergonic), ΔG > 0 is nonspontaneous, and ΔG = 0 is equilibrium. Spontaneous says no...

sp3d is the hybridization of a central atom with five electron domains, giving a trigonal bipyramidal electron geometry. It explains expanded octets in period 3 or lower, as in PCl₅. The textbook model uses one d orbital, though some acc...

sp³d² describes a central atom with six electron domains: one s, three p and two d orbitals mix into six equivalent hybrids pointing to the corners of an octahedron (90° apart). In the textbook model, steric number 6 means sp³d², as in S...

A lead-acid battery is a rechargeable battery with a Pb anode, a PbO₂ cathode and H₂SO₄ electrolyte, giving about 2 V per cell. On discharge both electrodes turn to PbSO₄ and the acid is consumed; recharging runs the reaction backward as...

One mole is the amount of a substance containing 6.022 × 10²³ elementary entities (atoms, molecules, ions, or formula units), a number called Avogadro's number. It is a count, not a mass: equal moles mean equal numbers of particles, and...

The azimuthal (angular momentum) quantum number, l, identifies an electron's subshell and the shape of its orbital. For a given n, l is an integer from 0 to n − 1, with l = 0, 1, 2, 3 labeled s, p, d, f. It also fixes the allowed ml valu...

A chemical bond is an attractive interaction that holds atoms together in a compound, lowering their total energy. The most testable point is classifying bonds (ionic, covalent, metallic) by electronegativity difference and predicting ho...

The kelvin (K) is the SI unit of absolute temperature, with 0 K at absolute zero. One kelvin is the same size as one Celsius degree, and T(K) = t(°C) + 273.15. Gas laws and kinetic theory need temperature in kelvins, not Celsius.

A molecular formula gives the actual number of atoms of each element in one molecule, such as C₆H₁₂O₆ for glucose. It is a whole-number multiple of the empirical formula, so you need the molar mass to get from the empirical formula to th...

A spectator ion is a dissolved ion that is present unchanged on both sides of a reaction, so it is left out of the net ionic equation. Testable point: in a precipitation or strong acid–strong base reaction, the ions that do not form the...

A voltaic (galvanic) cell uses a spontaneous redox reaction to produce electrical current. Oxidation occurs at the anode and reduction at the cathode, and E°cell is positive (ΔG° negative). Electrons flow from anode to cathode.

Chlorine (Cl, Z = 17) is a period 3 halogen with seven valence electrons. It readily gains one to form Cl⁻, so Cl₂ is a strong oxidizing agent. Testable point: Cl has a more negative electron affinity than F, even though F is more electr...

Dipole-dipole interactions are attractions between the partial positive end of one polar molecule and the partial negative end of another. For molecules of similar size and mass, they typically make boiling points higher than dispersion...

Ionization is the formation of ions from a neutral atom or molecule. For atoms, ionization energy is the energy needed to remove an electron from a gaseous atom; it generally rises across a period and falls down a group. In solution, mol...

Planck's constant, h = 6.63 × 10⁻³⁴ J·s, is the proportionality constant between a photon's energy and its frequency (E = hf). It shows that light and atomic energy levels are quantized. Photon energy depends on frequency, not on intensity.

The central cation is the positively charged metal ion at the center of a coordination complex, such as Cu²⁺ in [Cu(NH₃)₄]²⁺. It is a Lewis acid that accepts electron pairs from ligands. Testable point: the complex's net charge equals th...

A Lewis base is a species that donates a lone pair of electrons to form a bond with an electron-pair acceptor (a Lewis acid). The testable point: the definition does not require H⁺, so it is broader than the Brønsted definition, and ever...

A neutralization reaction is an acid reacting with a base to form a salt, and water in the Arrhenius case. It means the acid and base have consumed each other in stoichiometric amounts, not that the pH is 7. The pH at equivalence depends...

Dynamic equilibrium is the state in which the forward and reverse processes continue at equal rates, so the net concentrations of reactants and products stay constant. The most testable point: nothing stops at equilibrium, and concentrat...

Room temperature is the ambient lab temperature, roughly 20–25 °C. When no number is given, the MCAT usually means 25 °C (about 298 K), the temperature at which standard data are commonly tabulated. Convert to kelvin before any gas-law o...

The ground state is the lowest-energy state of an atom, with electrons in the lowest available orbitals (hydrogen: n = 1). Absorbing a photon that matches an energy gap lifts an electron to an excited state, and the atom emits that same...

A combustion reaction is the exothermic reaction of a fuel with O₂; complete burning of a hydrocarbon gives CO₂ and H₂O. The most testable points are balancing it (O last) and that ΔH°comb is negative, while limited O₂ gives CO or soot i...

An energy shell is the set of orbitals sharing the same principal quantum number n (1, 2, 3, …). Higher n means higher energy and a larger average distance from the nucleus. A shell holds up to 2n² electrons, and a jump between shells ab...

The heat of formation (standard enthalpy of formation, ΔH°f) is the enthalpy change when 1 mole of a compound forms from its elements in their standard states. Most testable point: ΔH°rxn = ΣΔH°f(products) − ΣΔH°f(reactants), and element...

The titrant is the solution of known concentration that is added gradually, usually from a buret, to the analyte of unknown concentration. Its known concentration and measured volume at the equivalence point give the moles of analyte.

Electrolysis uses an external power source to drive a nonspontaneous redox reaction in an electrolytic cell. Oxidation still occurs at the anode and reduction at the cathode, but the anode is positive and the cathode negative, the revers...

An acidic buffer is a solution of a weak acid and its conjugate base (such as acetic acid and acetate) that resists pH change and usually buffers below pH 7. It works best when pH is within about 1 unit of the pKa, and the pH is set by t...

Alkali metals are the Group 1 elements other than hydrogen: Li, Na, K, Rb, Cs and Fr. Each has one valence electron, so each forms a +1 cation. Going down the group, ionization energy falls and reactivity rises.

Autoionization of water is the reaction in which two water molecules transfer a proton to form H₃O⁺ and OH⁻: 2H₂O ⇌ H₃O⁺ + OH⁻. At 25 °C, Kw = [H⁺][OH⁻] = 1.0 × 10⁻¹⁴, so pure water has pH 7 and the product stays fixed in any aqueous sol...

A metal cation is a positively charged ion formed when a metal atom loses valence electrons, as in Na⁺, Ca²⁺ or Fe³⁺. It is smaller than its parent atom, and a higher positive charge on the same element gives a smaller ion.

A redox titration finds an unknown concentration by adding a standard oxidizing or reducing agent until the moles of electrons lost equal the moles gained (the equivalence point). The most testable point is to use the balanced equation's...

Standard enthalpy of formation (ΔH°f) is the enthalpy change when one mole of a compound forms from its elements in their standard states. It is zero for an element in its standard state. The MCAT uses it to find ΔH°rxn as products minus...

Total pressure is the pressure a gas mixture exerts on its container, equal to the sum of the partial pressures of its components (Dalton's law, exact for ideal gases). Each partial pressure is its mole fraction times the total pressure,...

Standard free energy ΔG° is the Gibbs free energy change for a reaction when all species are in their standard states (1 M solutes, 1 atm gases, usually 298 K). It sets the equilibrium constant: ΔG° = −RT ln Keq, so a negative ΔG° means...

Alkaline earth metals are the Group 2 elements (Be, Mg, Ca, Sr, Ba, Ra). They have two valence electrons (ns²) and typically form M²⁺ cations. Down the group, atomic radius and reactivity increase while ionization energy decreases.

A basic buffer is a weak base plus its conjugate acid (e.g., NH₃/NH₄⁺) that resists pH change by neutralizing added acid or added base. Testable point: pOH = pKb + log([conjugate acid]/[base]), so pOH = pKb when the two are equal.

In the Bohr model, orbits are fixed circular paths around the nucleus, each with a quantized energy set by n. An electron moving between orbits absorbs or emits a photon equal to the energy difference. Orbits are not orbitals, which are...

The kinetic theory of gases models a gas as particles in constant random motion that collide elastically and have negligible volume and no intermolecular forces. Its key result is that average kinetic energy is proportional to absolute t...

An amphoteric species can act as an acid or as a base, depending on what it reacts with. Water, HCO₃⁻, H₂PO₄⁻ and Al(OH)₃ are examples. The testable point is that the role depends on the partner: it donates H⁺ to a stronger base and acce...

Atomic size is the radius of an atom, often taken as half the distance between the nuclei of two bonded identical atoms. It generally increases down a group (more shells) and decreases left to right across a period (greater effective nuc...

Bromophenol blue is a weak-acid pH indicator that is yellow in its acid form and blue in its base form, changing color over roughly pH 3.0 to 4.6 (pKa near 4). It suits titrations whose equivalence point is acidic, such as a strong acid...

Methyl orange is a weak-acid pH indicator that is red below about pH 3.1 and yellow above about 4.4. Because its range is acidic, it suits titrations whose equivalence point is below 7, such as a strong acid titrated with a weak base.

A monatomic ion is a charged particle made from a single atom that has gained or lost electrons, such as Na⁺, Ca²⁺, Cl⁻ or O²⁻. Contrast it with a polyatomic ion like SO₄²⁻. Most testable: main-group charge follows from group position, a...

A polyatomic ion is a group of two or more covalently bonded atoms that carries a net charge, such as NO₃⁻, SO₄²⁻ or NH₄⁺. It behaves as one unit and generally stays intact when a salt dissolves, so fully dissociated Na₂SO₄ gives 2 Na⁺ a...