AP Intermediate Chemistry (BIEAP) key terms

The rule that electrons fill orbitals in order of increasing energy, starting with the lowest available level, so the ground-state configuration of an atom is built up orbital by orbital.

The rule that within a set of degenerate orbitals, electrons occupy separate orbitals with parallel spins before any pairing occurs, minimising electron-electron repulsion and giving maximum total spin.

The principle that no two electrons in an atom can have the same set of four quantum numbers, which limits each orbital to at most two electrons of opposite spin.

The set of four integers or half-integers (principal, azimuthal, magnetic, spin) that specifies the energy, shape, orientation and spin state of an electron in an atom.

A region of space around a nucleus where the probability of finding an electron is high, described mathematically by a wave function and holding a maximum of two electrons.

The net positive charge experienced by an outer electron after the shielding effect of inner electrons is subtracted from the actual nuclear charge; it governs atomic size and ionisation energy trends.

A measure of the size of an atom, usually taken as half the distance between nuclei of two bonded or touching atoms; it decreases across a period and increases down a group.

The minimum energy needed to remove the most loosely bound electron from an isolated gaseous atom in its ground state; it increases across a period and falls down a group.

The enthalpy change when one mole of gaseous atoms each accepts an electron to form a singly charged anion; a negative value means the process releases energy and is favourable.

The tendency of an atom in a molecule to attract the shared pair of electrons towards itself, commonly ranked on the Pauling scale; it increases across a period and decreases down a group.

The mixing of atomic orbitals of similar energy on the same atom to produce an equal number of new equivalent orbitals with identical shape and energy, explaining observed molecular geometries.

A model predicting molecular shape from the repulsion between electron pairs around a central atom, with lone pairs occupying more space than bonding pairs and so compressing bond angles.

The number of electron pairs shared between two atoms in a bond; in molecular orbital theory it equals half the difference between bonding and antibonding electrons.

A bonding model in which atomic orbitals combine to form molecular orbitals spread over the whole molecule, filling them by Aufbau order to predict bond order and magnetic behaviour.

A vector quantity equal to the product of charge and separation distance, measuring the polarity of a bond or molecule; it is zero for symmetrical molecules such as carbon dioxide.

A hypothetical gas whose molecules have negligible volume and exert no intermolecular forces, obeying the equation of state PV = nRT under all conditions.

The ratio Z = PV/nRT, which equals one for an ideal gas and deviates from one for a real gas, being less than one where attractive forces dominate and greater than one where repulsion dominates.

The highest temperature above which a substance cannot be liquefied however much pressure is applied; above it the gas and liquid phases merge into a single supercritical fluid.

The pressure a single component of a gas mixture would exert if it alone occupied the whole container at the same temperature; the total pressure is the sum of the partial pressures.

A state function equal to the internal energy plus the product of pressure and volume; the change in enthalpy equals the heat exchanged at constant pressure.

The statement that the enthalpy change of a reaction is the same whether it takes place in one step or several, because enthalpy is a state function independent of the route.

A thermodynamic state function measuring the dispersal of energy or disorder of a system; it increases in spontaneous processes for an isolated system, as stated by the second law.

A state function defined as G = H minus TS; its change at constant temperature and pressure is negative for a spontaneous process and zero at equilibrium.

The rule that when a system at equilibrium is disturbed by a change in concentration, pressure or temperature, the equilibrium shifts in the direction that counteracts the disturbance.

The ratio of the product of product concentrations to the product of reactant concentrations, each raised to its stoichiometric coefficient, at equilibrium; its value depends only on temperature.

The equilibrium constant for the dissolution of a sparingly soluble salt, equal to the product of the molar concentrations of its ions in saturated solution, each raised to its coefficient.

A solution that resists a change in pH on addition of small amounts of acid or base, usually made from a weak acid and its conjugate base or a weak base and its conjugate acid.

The potential of a half-cell measured relative to the standard hydrogen electrode under standard conditions of one molar concentration, one bar pressure and 298 K.

An equation relating the cell potential to standard potential and reaction quotient, E = E° minus (RT/nF) ln Q, showing how concentration changes shift the potential.

The maximum potential difference between the two electrodes of a galvanic cell when no current is drawn, equal to the difference between the cathode and anode reduction potentials.

An experimentally determined equation expressing the reaction rate as a product of a rate constant and the concentrations of reactants raised to powers known as orders.

The sum of the powers to which reactant concentrations are raised in the experimentally determined rate law; it may be zero, fractional or negative and is not fixed by stoichiometry.

The time taken for the concentration of a reactant to fall to half its initial value; for a first-order reaction it is independent of concentration and equals 0.693 divided by the rate constant.

An oxide that reacts with both acids and bases to form salts, such as aluminium oxide or zinc oxide, because it can act as either an acid or a base.

The similarity in chemical properties between certain elements placed diagonally in the periodic table, notably lithium with magnesium and beryllium with aluminium, arising from comparable charge-to-radius ratios.

The tendency of the outermost s electrons in heavy p-block elements to remain unshared, making the lower oxidation state two less than the group number more stable down the group.

An ion or neutral molecule that donates one or more lone pairs to a central metal ion to form a coordinate bond in a complex, classified as monodentate, bidentate or polydentate.

A model of coordination compounds that treats metal-ligand bonds as purely electrostatic, explaining colour and magnetism by the splitting of d orbitals in the ligand field.

The enhanced stability of complexes containing polydentate ligands compared with similar monodentate complexes, caused by the favourable entropy change when fewer particles bind the metal.

The permanent polarisation of a sigma bond caused by the electronegativity difference between adjacent atoms, transmitted along a chain and weakening with distance from the substituent.

The description of a molecule as a weighted average of two or more valid Lewis structures, called contributing structures, which differ only in the placement of pi and lone-pair electrons.

The stabilising interaction in which electrons of a sigma bond, usually a carbon-hydrogen bond adjacent to a pi system, delocalise into an empty or partially filled p orbital.

The rule that when a hydrogen halide adds to an unsymmetrical alkene, the hydrogen attaches to the carbon bearing more hydrogen atoms, giving the more stable carbocation intermediate.

A positively charged carbon species with only three bonds and an empty p orbital; its stability increases from primary to secondary to tertiary and is enhanced by resonance or hyperconjugation.

A chain reaction of alkanes with halogens in ultraviolet light, involving initiation to form radicals, propagation to give the haloalkane and regeneration of radicals, and termination.

A reaction in which a nucleophile replaces a leaving group on a saturated carbon, proceeding by a one-step SN2 mechanism or a carbocation-based SN1 mechanism depending on substrate and conditions.

A reaction in which an enolate formed from an aldehyde or ketone attacks the carbonyl carbon of another molecule, giving a beta-hydroxy carbonyl compound that may dehydrate to an alpha,beta-unsaturated product.

A disproportionation of aldehydes lacking alpha hydrogens, such as benzaldehyde, in strong alkali, producing equimolar amounts of the corresponding alcohol and carboxylate salt.

A test for primary amines in which the amine is heated with chloroform and alcoholic potassium hydroxide to give an isocyanide with a characteristic foul smell.