CAPE Chemistry Unit 1 key terms

The weighted average mass of the naturally occurring atoms of an element compared with one-twelfth the mass of a carbon-12 atom. It is dimensionless because it is a ratio of masses.

The sum of the relative atomic masses of all atoms in one molecule of a covalent substance, expressed relative to one-twelfth the mass of a carbon-12 atom. It is a ratio and has no units.

One of two or more atoms of the same element whose nuclei contain equal numbers of protons but different numbers of neutrons, so they share chemical behaviour but differ in mass and sometimes in nuclear stability.

The amount of substance that contains exactly the same number of elementary entities as there are atoms in twelve grams of carbon-12, that number being the Avogadro constant.

The number of elementary entities per mole of substance, approximately 6.02 x 10 to the power 23 per mole. It links the microscopic count of particles to measurable macroscopic amounts.

The mass of one mole of a substance in grams per mole, numerically equal to its relative molecular or formula mass. It converts between mass in grams and amount in moles.

The simplest whole-number ratio of the atoms of each element present in a compound, obtained by dividing the mole ratio of the elements by the smallest value.

The actual number of atoms of each element in one molecule of a compound. It is a whole-number multiple of the empirical formula and needs molar mass data to be deduced.

The reactant that is completely consumed first in a chemical reaction, thereby fixing the maximum quantity of product that can be formed and leaving other reactants in excess.

The actual mass of product obtained expressed as a percentage of the theoretical maximum mass calculated from the stoichiometry of the balanced equation, accounting for losses and side reactions.

The quantitative relationship between the amounts of reactants and products in a balanced chemical equation, allowing masses, moles and volumes to be calculated by simple proportion.

The volume occupied by one mole of any gas under specified conditions, approximately 24 cubic decimetres per mole at room temperature and pressure for an ideal gas.

The relationship pV equals nRT linking pressure, volume, amount and absolute temperature of a gas through the universal gas constant, used to find any one variable when the others are known.

An analytical technique that ionises a sample, separates the ions by their mass-to-charge ratio and records their relative abundances, allowing relative atomic mass and molecular structure to be determined.

A region of space around a nucleus in which the probability of finding an electron is high. Each orbital holds at most two electrons of opposite spin.

The arrangement of electrons among the orbitals of an atom or ion, written to show the subshells occupied and the number of electrons in each, consistent with the Aufbau order.

The rule that electrons occupy the lowest available energy orbitals first when building up the configuration of an atom, filling orbitals in order of increasing energy.

The rule that electrons occupy degenerate orbitals singly with parallel spins before any pairing occurs, minimising electron-electron repulsion within a subshell.

The minimum energy needed to remove one mole of electrons from one mole of gaseous atoms of an element, producing gaseous singly positive ions. It indicates how easily an atom loses electrons.

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 distance from the nucleus to the outer electron shell.

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, producing a giant lattice structure.

A bond formed by the sharing of one or more pairs of electrons between two atoms, with the shared pair attracted to both nuclei and holding the atoms together.

A covalent bond in which both electrons of the shared pair are supplied by one atom, as in the formation of an ammonium ion from ammonia and a hydrogen ion.

A covalent bond in which the shared electrons are unevenly distributed because the bonded atoms have different electronegativities, creating partial positive and partial negative charges.

A pair of valence electrons in an atom that is not involved in bonding. Lone pairs influence molecular shape and can act as electron donors in dative bonds.

A model predicting the shape of a molecule from the repulsion between electron pairs around a central atom, which arrange themselves as far apart as possible so that lone pairs repel more strongly than bonding pairs.

The angle between two covalent bonds meeting at a central atom, determined by the arrangement of electron pairs and used as evidence for molecular shape and hybridisation.

A measure of the polarity of a bond or molecule, equal to the product of charge and separation distance. Symmetrical molecules can have polar bonds yet a zero overall dipole moment.

A relatively weak attractive force between separate molecules, such as instantaneous dipole, permanent dipole or hydrogen bonding, which affects melting and boiling points but is much weaker than a covalent bond.

An unusually strong intermolecular attraction between a hydrogen atom bonded to a highly electronegative atom and a lone pair on a neighbouring electronegative atom such as oxygen, nitrogen or fluorine.

Weak intermolecular attractions arising from temporary or permanent dipoles, including instantaneous dipole-induced dipole forces that increase in strength with the number of electrons and molecular size.

The electrostatic attraction between a lattice of positive metal ions and a sea of delocalised valence electrons, which explains the electrical conductivity and malleability of metals.

An electron that is not confined to a single bond or atom but is spread over several atoms, as in metals, benzene or a carbonate ion, giving extra stability and conductivity.

A description of a molecule as a weighted average of two or more valid Lewis structures, used when a single structure cannot account for the observed equal bond lengths and delocalised bonding.

The number of moles of one component divided by the total number of moles of all components in a mixture. The mole fractions of all components sum to one.

The amount of solute in moles dissolved per cubic decimetre of solution, usually expressed in moles per cubic decimetre and used in titration and equilibrium calculations.

A volumetric procedure in which a solution of known concentration is added from a burette to a measured volume of another solution until the equivalence point is reached, allowing unknown concentration to be calculated.

A number assigned to an atom in a species that tracks the electrons lost or gained, with the convention that oxidation is an increase and reduction a decrease in this value.

A reaction in which one species loses electrons and is oxidised while another gains electrons and is reduced, so oxidation and reduction always occur together.

A redox reaction in which the same element in a single species is simultaneously oxidised and reduced, producing products with higher and lower oxidation numbers.