Cameroon GCE A-Level Chemistry key terms

The weighted mean mass of the atoms of an element compared with one-twelfth of the mass of a carbon-12 atom. It accounts for the natural abundances of the element's isotopes.

Atoms of the same element with the same number of protons but different numbers of neutrons, hence different mass numbers. Isotopes have identical chemical properties but may differ in physical behaviour and stability.

The total number of protons and neutrons in the nucleus of an atom. It is written as a superscript before the symbol and distinguishes isotopes of the same element.

The energy needed to remove one mole of electrons from one mole of gaseous atoms or ions. Successive values increase as each electron is taken from an increasingly positive species.

The regular repetition of physical and chemical properties of elements across periods and down groups, caused by recurring outer electron configurations. It explains trends in atomic radius, ionisation energy and reactivity.

A measure of the tendency of an atom in a covalent bond to attract the shared pair of electrons towards itself. Values rise across a period and fall down a group.

Electrostatic attraction between oppositely charged ions formed by electron transfer from a metal to a non-metal. The resulting giant lattice gives high melting points and conducts when molten or aqueous.

A bond formed when two atoms share one or more pairs of electrons, held together by attraction to both nuclei. It may be single, double or triple, and may be polar or non-polar.

A covalent bond in which the shared electrons are unevenly distributed because the bonded atoms have different electronegativities, producing partial charges. Polar bonds can make a molecule polar overall if the dipoles do not cancel.

A strong intermolecular attraction between a lone pair on a highly electronegative atom (N, O or F) and a hydrogen atom bonded to another such atom. It explains water's high boiling point and ice's low density.

Attractive forces between separate molecules, including London dispersion, dipole–dipole and hydrogen bonding. They are much weaker than covalent bonds but control melting and boiling points.

A hypothetical gas whose particles occupy negligible volume and exert no intermolecular forces, obeying pV = nRT exactly. Real gases approach ideal behaviour at low pressure and high temperature.

The volume occupied by one mole of a gas under stated conditions, about 24 dm³ at room temperature and pressure. It allows volumes of gases to be converted to amounts in moles.

The amount of substance containing as many elementary entities as there are atoms in exactly 12 g of carbon-12, namely the Avogadro constant. It links mass, volume and concentration calculations.

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions. Values are used with Hess's law to find unknown enthalpy changes.

The total enthalpy change of a reaction is independent of the route taken, provided the initial and final conditions are the same. It allows enthalpy changes to be calculated from formation or combustion data.

The heat energy transferred at constant pressure during a chemical reaction, given the symbol ΔH. Negative values indicate exothermic change, positive values endothermic change.

The minimum energy that colliding particles must possess for a reaction to occur. A lower activation energy, achieved with a catalyst, increases the fraction of successful collisions.

When a system at equilibrium is disturbed by a change in conditions, the position of equilibrium shifts to oppose that change. It predicts effects of concentration, pressure and temperature changes.

A reversible reaction in which the forward and reverse rates are equal, so concentrations of reactants and products remain constant although both reactions continue. It requires a closed system.

The ratio of product concentrations to reactant concentrations, each raised to its stoichiometric power, for a reaction at equilibrium at a stated temperature. It changes only with temperature.

A species that donates a proton (H⁺) to another species. Its conjugate base is formed when it loses that proton, giving rise to conjugate acid–base pairs.

Two species differing by one proton, such as CH₃COOH and CH₃COO⁻. The stronger an acid, the weaker its conjugate base.

A solution that resists changes in pH when small amounts of acid or alkali are added, usually made from a weak acid and its conjugate base. It works because the components remove added H⁺ or OH⁻.

The negative logarithm to base ten of the hydrogen ion concentration, pH = −log₁₀[H⁺]. A change of one pH unit corresponds to a tenfold change in hydrogen ion concentration.

A process in which a species loses electrons, and its oxidation number increases. Oxidation is always accompanied by reduction in a redox reaction.

A reaction in which electrons are transferred from one species to another, so oxidation and reduction occur together. Oxidation numbers of some atoms change during the reaction.

A process in which a species gains electrons, and its oxidation number decreases. The oxidising agent is the species that accepts the electrons and is itself reduced.

An ion or molecule that donates one or more lone pairs to a central metal ion to form a coordinate bond in a complex. Common examples include water, ammonia and chloride.

A charged species consisting of a central metal ion surrounded by ligands bonded by coordinate bonds. Its colour arises from electron transitions between split d orbitals.

The number of coordinate bonds from ligands to the central metal ion in a complex. It is commonly 4 or 6, giving tetrahedral, square planar or octahedral shapes.

A substance that increases the rate of a reaction by providing an alternative pathway of lower activation energy, without being used up. It does not change the position of equilibrium.

The change in concentration of a reactant or product per unit time. It is measured by following a physical property such as gas volume, mass loss or colour intensity.

One of two or more compounds with the same molecular formula but different arrangements of atoms, giving different physical and chemical properties. They may have different positions of a functional group or different chain branching.

A family of organic compounds with the same functional group and similar chemical properties, in which successive members differ by CH₂. Members show a gradual trend in physical properties.

The specific group of atoms in an organic molecule that determines its characteristic chemical reactions. Examples include the hydroxyl, carbonyl, carboxyl and amino groups.

A reaction in which a nucleophile donates an electron pair to an electron-deficient carbon atom, displacing a leaving group such as a halide ion. Primary halogenoalkanes favour an SN2 mechanism.

An electron-deficient species that accepts an electron pair from a nucleophile or a double bond. Positively charged ions and polar molecules can act as electrophiles in addition reactions.

A species with a lone pair of electrons that donates it to an electron-deficient atom, forming a new bond. Examples include hydroxide ions, ammonia and cyanide ions.

A quantitative procedure in which a solution of known concentration is added from a burette to a measured volume of another until the end-point is reached. It allows the unknown concentration to be calculated.