NSSCAS Chemistry (8224) key terms

The weighted average mass of the atoms of an element compared with one-twelfth of the mass of a carbon-12 atom, taking natural isotopic abundances into account.

Atoms of the same element with the same number of protons but different numbers of neutrons, so they share chemical properties but differ in mass and sometimes in nuclear stability.

The total number of protons and neutrons in the nucleus of a particular atom, written as a superscript before the element symbol.

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

The energy needed to remove one mole of electrons from one mole of gaseous atoms of an element to form one mole of gaseous 1+ ions.

A measure of the size of an atom, usually half the distance between the nuclei of two identical atoms joined by a single bond, or a calculated value from electron density.

The tendency of an atom in a covalent bond to attract the shared pair of electrons towards itself; values increase across a period and decrease down a group.

The electrostatic attraction between oppositely charged ions formed when electrons are transferred from a metal atom to a non-metal atom.

A bond formed by the sharing of a pair of electrons between two atoms, with each atom contributing one electron to the shared pair.

A covalent bond in which both electrons of the shared pair come from the same atom, as in the bond between nitrogen and boron in an ammonium ion.

A relatively weak attraction between separate molecules, such as instantaneous dipole–induced dipole forces or hydrogen bonding, which affects melting and boiling points.

A strong intermolecular attraction between a hydrogen atom bonded to N, O or F and a lone pair on a nearby N, O or F atom of another molecule.

A covalent bond in which the electron density is unevenly shared because the two bonded atoms have different electronegativities, giving partial charges.

The amount of substance containing the same number of elementary entities as there are carbon atoms in exactly 12 g of carbon-12, namely the Avogadro constant.

The mass of one mole of a substance, expressed in grams per mole and numerically equal to its relative molecular or formula mass.

The sum of the relative atomic masses of all the atoms in one molecule of a covalent substance, or in the formula unit of an ionic compound.

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

The actual number of atoms of each element in one molecule of a compound; it is a whole-number multiple of the empirical formula.

The reactant that is completely used up first in a reaction and therefore determines the maximum amount of product that can be formed.

The actual mass or amount of product obtained expressed as a percentage of the theoretical maximum calculated from the balanced equation.

The molar mass of the desired product divided by the total molar mass of all reactants, expressed as a percentage; it measures how efficiently atoms end up in the target product.

The heat energy transferred at constant pressure when a reaction occurs in the molar quantities shown in the equation, with a negative sign for exothermic reactions.

The enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions of 298 K and 100 kPa.

The total enthalpy change of a reaction is independent of the route taken, provided the initial and final conditions are the same; used to calculate enthalpy changes indirectly.

The average energy required to break one mole of a particular covalent bond in the gaseous state, always an endothermic positive value.

The minimum energy that colliding particles must possess for a reaction to occur, corresponding to the energy needed to reach the transition state.

A substance that increases the rate of a reaction by providing an alternative pathway of lower activation energy, without being used up in the overall reaction.

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

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

When a system at equilibrium is disturbed by a change in concentration, pressure or temperature, the position of equilibrium shifts to oppose the change.

The ratio of the equilibrium concentrations of products to reactants, each raised to the power of its stoichiometric coefficient, at a fixed temperature.

A species that donates a proton (hydrogen ion) to another species; a base accepts a proton, and the two differ by one H+.

Two species that differ by the loss or gain of a single proton, such as NH3 and NH4+, or CH3COOH and CH3COO−.

A logarithmic measure of hydrogen ion concentration, defined as the negative logarithm to base ten of the hydrogen ion concentration in mol dm−3.

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.

Loss of electrons by a species, accompanied by an increase in oxidation number; in organic chemistry it may also mean gain of oxygen or loss of hydrogen.

Gain of electrons by a species, accompanied by a decrease in oxidation number; in organic chemistry it may also mean gain of hydrogen or loss of oxygen.

A number assigned to an element in a compound that reflects the number of electrons lost or gained, calculated using agreed rules for each atom.

The potential of a half-cell measured relative to the standard hydrogen electrode under standard conditions, used to predict the direction of redox reactions.

A specific atom or group of atoms within a molecule that gives the compound its characteristic chemical reactions and physical properties.

A family of organic compounds with the same general formula and functional group, showing a gradual change in physical properties and similar chemical behaviour.

Compounds with the same molecular formula but different arrangements of atoms, leading to different physical or chemical properties.

A molecule that has the same structural formula as another but a different spatial arrangement, as in E/Z isomers or optical isomers.

An electron-rich species that donates a pair of electrons to an electron-deficient carbon atom, attacking a δ+ centre in substitution or addition reactions.

An electron-deficient species that accepts a pair of electrons from an electron-rich region, often attacking a double bond in addition reactions.

A procedure in which a solution of known concentration is added from a burette to a measured volume of another solution until the reaction is just complete, indicated by an indicator or instrument.

A solution of accurately known concentration, prepared by dissolving a weighed mass of pure substance in a volumetric flask and making up to the mark.

The point in a titration at which the indicator changes colour, ideally matching the equivalence point where the reactants have reacted in the mole ratio of the equation.

A separation technique in which components move at different rates between a stationary phase and a mobile phase, allowing identification by comparison with reference Rf values.

The distance travelled by a component divided by the distance travelled by the solvent front in thin-layer or paper chromatography; a value between 0 and 1.