UEE Chemistry key terms

The number of protons in the nucleus of an atom, which uniquely identifies the element and determines the number of electrons in a neutral atom.

The total number of protons and neutrons in a nucleus, giving the approximate relative atomic mass of that particular nuclide.

Atoms of the same element with identical proton numbers but different neutron numbers, so they share chemical behaviour but differ in mass and sometimes nuclear stability.

The weighted average mass of an element's atoms compared with one-twelfth the mass of a carbon-12 atom, accounting for natural isotopic abundance.

The minimum energy needed to remove one mole of electrons from one mole of gaseous atoms or ions, measured in successive stages and revealing shell structure.

A measure of the size of an atom, typically half the distance between neighbouring nuclei in an element; it decreases across a period and increases down a group.

The tendency of an atom in a bond to attract the shared electron pair towards itself; values rise across a period and fall down a group.

A region of defined energy around a nucleus; shells contain subshells labelled s, p, d and f, each holding orbitals of characteristic shape and capacity.

A region of space where the probability of finding an electron is high; each orbital holds at most two electrons of opposite spin.

Rules for filling orbitals: electrons occupy the lowest available energy level first and singly fill degenerate orbitals with parallel spins before pairing up.

The electrostatic attraction between oppositely charged ions formed when electrons are transferred from a metal to a non-metal, producing a giant lattice.

A bond formed by the sharing of one or more pairs of electrons between atoms, giving a directional linkage with a specific bond length and energy.

A covalent bond in which the shared electrons are drawn towards the more electronegative atom, creating partial charges and often a permanent dipole.

A model that predicts molecular shape by arranging electron pairs around a central atom so that repulsions between them are minimised, treating lone pairs as more repulsive.

The mixing of atomic orbitals on a central atom to give equivalent hybrid orbitals (sp, sp2, sp3) whose arrangement explains observed bond angles and geometry.

Attractions between separate molecules, including London dispersion, permanent dipole-dipole and hydrogen bonding, which are much weaker than covalent or ionic bonds.

A strong dipole-dipole attraction between a hydrogen atom bonded to N, O or F and a lone pair on a nearby electronegative atom, raising boiling points markedly.

The amount of substance containing 6.02 × 10^23 specified particles, linking mass, molar mass and the number of entities in a sample.

The number of particles in one mole, approximately 6.02 × 10^23 per mole, used to convert between particle counts and amount of substance.

The mass of one mole of a substance in grams per mole, numerically equal to its relative formula mass and used to convert mass to amount.

The simplest whole-number ratio of atoms of each element in a compound, obtained from percentage composition or combustion data.

The reactant that is completely consumed first in a reaction, fixing the maximum amount of product that can form according to the balanced equation.

The relationship pV = nRT linking pressure, volume, amount and absolute temperature, with R equal to 8.31 J mol⁻¹ K⁻¹.

The pressure a single gas in a mixture would exert if it alone occupied the container; total pressure is the sum of partial pressures.

The volume occupied by one mole of any gas under stated conditions, about 24 dm³ at room temperature and pressure, used in gas calculations.

The heat energy transferred at constant pressure during a reaction, given the symbol ΔH and negative for exothermic processes.

The enthalpy change when one mole of a compound forms from its elements in their standard states under standard conditions, usually measured in kJ mol⁻¹.

The total enthalpy change of a reaction is independent of the route taken, allowing unknown enthalpy changes to be calculated from known ones in a cycle.

The average energy required to break one mole of a particular bond in gaseous molecules, used to estimate reaction enthalpy changes.

The state in a reversible reaction where forward and reverse rates are equal, so concentrations remain constant although both reactions continue.

The ratio of product concentrations raised to their stoichiometric powers to reactant concentrations similarly raised, at equilibrium and at a fixed temperature.

When a system at equilibrium is disturbed by a change in concentration, pressure or temperature, it shifts in the direction that opposes the change.

An acid is a proton donor and a base is a proton acceptor; every acid has a conjugate base formed after losing a proton.

The negative logarithm to base ten of the hydrogen ion concentration in mol dm⁻³, giving a scale from acidic to alkaline solutions.

A mixture of a weak acid and its conjugate base, or a weak base and its conjugate acid, that resists pH change on adding small amounts of acid or alkali.

The change in concentration of a reactant or product per unit time, often measured by monitoring volume, mass, colour or conductivity.

The minimum kinetic energy colliding particles need to overcome the energy barrier and react; it corresponds to the peak of the energy profile.

A substance that increases reaction rate by providing an alternative pathway of lower activation energy without being consumed overall.

Oxidation is loss of electrons and reduction is gain of electrons; in redox reactions the two always occur together and oxidation numbers change.

The potential of a half-cell measured against the standard hydrogen electrode under standard conditions, indicating relative tendency to be reduced.

The decomposition of an electrolyte by passing a direct current through it, with oxidation at the anode and reduction at the cathode.

An atom or group of atoms within a molecule that determines its characteristic reactions, such as hydroxyl, carbonyl, carboxyl or amino groups.

A family of organic compounds with the same functional group and general formula, differing by CH2 units and showing gradual trends in physical properties.

The existence of compounds with the same molecular formula but different arrangements of atoms, giving structural or stereoisomers with distinct properties.

A reaction in which an electron-rich species replaces a leaving group on a carbon atom, typical of halogenoalkanes reacting with hydroxide or cyanide ions.

A reaction in which an electron-deficient species attacks a double bond, forming a carbocation intermediate and adding across the alkene.

A volumetric technique in which a solution of known concentration is added from a burette to a measured volume of analyte until the equivalence point is signalled.

A solution whose concentration is accurately known, prepared by dissolving an exact mass of a pure solute and making up to a calibrated volume.

A weak acid or base whose colour changes over a specific pH range, chosen so the end point coincides with the equivalence point of a titration.

A purification method in which a solid is dissolved in a hot solvent, filtered, then cooled so pure crystals form while impurities remain in solution.