IB Chemistry SL key terms

The amount of substance that contains exactly 6.022 × 10²³ specified particles. It lets chemists count atoms and molecules by weighing, because one mole of any substance has a mass in grams equal to its relative mass.

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 and has no units.

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

The simplest whole-number ratio of atoms of each element in a compound. It is found from percentage composition or from combustion data, and may differ from the molecular formula.

A region of space around the nucleus where the probability of finding an electron is high. Each orbital holds at most two electrons of opposite spin and has a characteristic shape and energy.

The arrangement of electrons among the orbitals of an atom, written to show the principal level and subshell, for example 1s² 2s² 2p⁴. It explains chemical properties and position in the periodic table.

The energy needed to remove one mole of electrons from one mole of gaseous atoms to form one mole of gaseous 1+ ions. It reflects nuclear charge, atomic radius and electron shielding.

The electrostatic attraction between oppositely charged ions in a lattice, formed when electrons are transferred from a metal to a non-metal. Its strength depends on ionic charge and ionic radius.

A bond formed when two atoms share one or more pairs of electrons. The shared pair is attracted to both nuclei, and the bond length and strength depend on the atoms involved.

The attraction between a lattice of positive metal ions and a sea of delocalised valence electrons. It explains electrical and thermal conductivity, malleability and generally high melting points.

A measure of the tendency of an atom in a molecule to attract the shared electrons of a covalent bond towards itself. It increases across a period and decreases down a group.

A strong form of dipole–dipole attraction between a hydrogen atom bonded to N, O or F and a lone pair on a nearby N, O or F atom. It explains water's unusually high boiling point.

Temporary attractive forces caused by instantaneous and induced dipoles in electron clouds. They increase with the number of electrons and with molecular surface area, so larger molecules have stronger dispersion forces.

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

The regular, repeating variation in physical and chemical properties of the elements across a period and down a group. It arises from changes in nuclear charge, atomic radius and electron shielding.

A d-block element that forms at least one stable ion with an incompletely filled d subshell. Such elements typically show variable oxidation states, coloured compounds and catalytic activity.

A family of organic compounds sharing the same functional group and general formula, in which each member differs from the next by a CH₂ unit. Physical properties change gradually along the series.

An atom or group of atoms within a molecule that is responsible for its characteristic chemical reactions, such as the –OH group in alcohols or the –COOH group in carboxylic acids.

Compounds with the same molecular formula but different connections between their atoms. This different connectivity gives them different physical properties and often different chemical behaviour.

The enthalpy change when one mole of a substance is completely burned in excess oxygen, with all reactants and products in their standard states at 100 kPa and a stated temperature, usually 298 K.

The enthalpy change for a reaction depends only on the initial and final states, not on the route taken. It allows unknown enthalpy changes to be calculated from known ones.

The average energy required to break one mole of a particular bond in the gaseous state. Values are averages over different compounds, so enthalpy changes calculated from them are approximate.

The minimum kinetic energy that colliding particles must have for a successful reaction. It corresponds to the energy barrier between the reactants and the transition state.

The reactant that is used up first in a reaction because it is present in the smaller amount relative to the stoichiometric ratio. It determines the maximum amount of product that can form.

The model stating that particles must collide with enough energy, and in a suitable orientation, for a reaction to occur. It explains the effects of concentration, temperature, surface area and catalysts on rate.

A substance that speeds up a reaction by providing an alternative route with lower activation energy without being used up. It does not change the equilibrium position or the enthalpy change.

The change in concentration of a reactant or product per unit time, typically in mol dm⁻³ s⁻¹. It is found experimentally, for example from the gradient of a concentration–time graph.

The state of a closed system in which the forward and reverse reactions proceed at equal rates, so the concentrations of reactants and products stay constant even though both reactions continue.

When a change is made to a system at equilibrium, the position of equilibrium shifts in the direction that tends to oppose the change, as far as possible.

A species that donates a proton (H⁺ ion) to another species. When it loses the proton it becomes its conjugate base, and the two differ by one hydrogen ion.

The negative base-ten logarithm of the hydrogen ion concentration measured in mol dm⁻³. A fall of one pH unit means the hydrogen ion concentration has increased tenfold.

A number assigned to an element in a compound showing the charge it would have if bonding were fully ionic. Oxidation is an increase in oxidation number and reduction is a decrease.

The potential of a half-cell measured against the standard hydrogen electrode under standard conditions, which is set at 0.00 V. More positive values indicate a greater tendency to be reduced.

An electron-rich species with a lone pair that it donates to an electron-deficient atom to form a new bond, as in the substitution of a halogenoalkane.

An electron-deficient species that accepts a lone pair from an electron-rich region to form a new bond, for example when it is added across a carbon–carbon double bond.

The slowest step in a multi-step reaction mechanism. It controls the overall rate, and only species involved in this step appear in the experimental rate equation.

The range within which the true value of a measurement is likely to lie, reflecting the limits of the instrument and technique, for example ± half the smallest division on an analogue scale.

An error that shifts every measurement in the same direction by a similar amount, caused by faulty equipment or method. It affects accuracy and is not removed by averaging repeats.

Unpredictable variation between repeated measurements caused by factors such as reading fluctuations. It affects precision and can be reduced by taking repeats and averaging the results.