GTLE Subject Specialism Chemistry key terms

The net positive charge an outer electron experiences after inner-shell electrons screen the nucleus. It increases across a period, pulling electrons closer and raising ionization energy and electronegativity.

A measure of the size of an atom, usually half the distance between nuclei of two bonded or touching atoms. It decreases across a period as Zeff rises and increases down a group as shells are added.

The minimum energy required to remove one electron from a gaseous atom or ion. Successive values reveal shell structure; large jumps indicate removal from a more tightly bound inner shell.

The energy change when a gaseous atom gains an electron to form a singly charged anion. A more negative value means the atom accepts electrons more readily, as with halogens.

The relative ability of an atom in a bond to attract shared electrons toward itself. Differences between bonded atoms predict bond polarity and, for metals with nonmetals, ionic character.

A model predicting molecular shape by arranging electron domains around a central atom as far apart as possible. Lone pairs occupy more space than bonding pairs and compress bond angles.

The mixing of atomic orbitals on a central atom to form equivalent orbitals that match observed geometry, such as sp3 for tetrahedral and sp2 for trigonal planar arrangements.

A vector quantity equal to charge separation multiplied by distance, summed over all bonds and lone pairs. A nonzero molecular dipole requires polar bonds arranged asymmetrically.

A strong intermolecular attraction between a hydrogen bonded to N, O, or F and a lone pair on another electronegative atom. It raises boiling points and shapes biomolecules.

Weak, transient attractions caused by instantaneous fluctuations in electron distribution. They grow with polarizability and contact area, so larger molecules have stronger dispersion forces.

The amount of substance containing exactly Avogadro's number of elementary entities, 6.022 x 10^23. It links countable particles to measurable masses through molar mass.

The reactant consumed first in a reaction, which fixes the maximum amount of product. Excess reactants remain after the reaction stops.

The maximum mass or amount of product calculated from stoichiometry when the limiting reactant is fully consumed. Percent yield compares actual product to this value.

The simplest whole-number ratio of atoms of each element in a compound. It is found by converting percentage composition or masses to moles and dividing by the smallest value.

Concentration expressed as moles of solute per liter of solution. It is temperature dependent because solution volume changes with temperature, unlike molality.

The equation PV = nRT relating pressure, volume, amount, and absolute temperature of a gas through the gas constant R. It assumes negligible particle volume and no intermolecular forces.

A model treating gas particles as point masses in constant random motion whose collisions are elastic. Average kinetic energy depends only on absolute temperature, not on gas identity.

The pressure a single gas in a mixture would exert if it alone occupied the container. By Dalton's law, the total pressure equals the sum of all partial pressures.

The heat exchanged at constant pressure during a process. Negative values indicate exothermic reactions releasing heat; positive values indicate endothermic reactions absorbing heat.

The enthalpy change of an overall reaction is the sum of enthalpy changes for any sequence of steps adding to it, because enthalpy is a state function.

The enthalpy change when one mole of a compound forms from its elements in their standard states at defined conditions. Values are zero for elements in their standard state.

A thermodynamic potential given by G = H - TS. A negative change in G indicates a spontaneous process at constant temperature and pressure.

A measure of the dispersal of energy among accessible microstates. It increases for gases over liquids over solids and generally rises with temperature and particle number.

The ratio of product activities to reactant activities at equilibrium, each raised to its stoichiometric coefficient. Its size indicates whether products or reactants dominate at equilibrium.

When a system at equilibrium is disturbed by changes in concentration, pressure, or temperature, it shifts in the direction that partially offsets the disturbance.

The minimum energy colliding particles need to convert into products. It determines the temperature dependence of rate and is lowered by a catalyst.

The slowest elementary step in a reaction mechanism. It limits the overall rate, so its reactants appear in the experimental rate law.

A species that donates a proton to another species. Its conjugate base is what remains after the proton is lost.

The negative base-10 logarithm of hydrogen ion activity, commonly approximated as -log[H+]. Values below 7 are acidic at standard conditions, above 7 basic.

A solution resisting pH change on addition of small amounts of acid or base, made from a weak acid and its conjugate base. Capacity depends on concentrations of both components.

A formal charge assigned to an atom assuming bonding electrons go to the more electronegative partner. It tracks electron transfer in redox reactions.

The potential of a half-cell measured against the standard hydrogen electrode under standard conditions. More positive values indicate greater tendency to be reduced.

An atom or group of atoms within a molecule that determines characteristic chemical behavior, such as hydroxyl, carbonyl, carboxyl, amine, or ester groups.

The existence of compounds with the same molecular formula but different arrangements or spatial orientations of atoms, giving distinct physical and chemical properties.

An electron-rich species that donates an electron pair to an electron-poor center. Common examples include hydroxide, cyanide, and ammonia.

Two substitution pathways: SN1 proceeds through a carbocation intermediate with first-order kinetics, while SN2 is a single concerted backside attack with second-order kinetics and inversion.

An ion or molecule that donates one or more electron pairs to a central metal ion, forming a coordination complex. Common ligands include water, ammonia, and chloride.

The number of donor atoms directly bonded to the central metal in a complex. It is commonly six for octahedral and four for tetrahedral or square planar complexes.