SPM Chemistry key terms

Anything that has mass and takes up space. Matter exists in three common states - solid, liquid and gas - and is built from tiny particles that are always moving.

The model that explains the behaviour of matter: particles are in constant random motion, and their kinetic energy rises with temperature, which explains diffusion, gas pressure and changes of state.

A physical change such as melting, boiling, freezing, condensation or sublimation, in which particles gain or lose energy but the substance stays chemically the same.

A pure substance made of only one kind of atom, such as iron or oxygen. An element cannot be broken down into simpler substances by ordinary chemical reactions.

A pure substance formed when two or more elements combine chemically in a fixed ratio, for example water (H₂O). A compound has properties different from those of its constituent elements.

Two or more substances combined physically rather than chemically, in any proportion. Each substance keeps its own properties, and the mixture can be separated by physical methods such as filtration or distillation.

The number of protons in the nucleus of an atom. It is also called the atomic number, and it decides which element the atom belongs to and its position in the Periodic Table.

The total number of protons and neutrons in a nucleus, also called the mass number. Subtracting the proton number from it gives the number of neutrons.

Atoms of the same element with the same number of protons but different numbers of neutrons, so they have identical chemical properties but different masses, for example carbon-12 and carbon-14.

The way electrons are arranged in shells around a nucleus, filled from the innermost shell outwards, for example 2.8.1 for sodium. It explains an element's group and period.

An electron in the outermost occupied shell of an atom. Valence electrons take part in bonding, and for main-group elements their number equals the group number.

A vertical column of the Periodic Table. Elements in the same group have the same number of valence electrons and therefore similar chemical properties that change gradually down the column.

A horizontal row of the Periodic Table. Across a period the proton number increases by one from element to element, and atoms of the same period have the same number of occupied shells.

A metal in the block between Groups 2 and 13, such as iron or copper. Transition elements show variable oxidation numbers, form coloured compounds and often act as catalysts.

The strong electrostatic attraction between oppositely charged ions. It forms when a metal atom transfers electrons to a non-metal atom, as in sodium chloride.

A bond formed when two atoms share one or more pairs of electrons, normally between non-metal atoms. The shared pair is attracted to both nuclei and holds them together.

The attraction between a lattice of positive metal ions and the delocalised electrons moving between them. These free electrons explain electrical conduction, malleability and the high melting points of metals.

Weak attractive forces between separate molecules, much weaker than covalent bonds. They must be overcome when a molecular substance melts or boils.

The chemical formula that shows the simplest whole-number ratio of atoms of each element in a compound, for example CH₂O for glucose.

The chemical formula that shows the actual number of atoms of each element in one molecule, for example C₆H₁₂O₆. It is a whole-number multiple of the empirical formula.

The amount of substance that contains 6.02 × 10²³ particles, a value known as Avogadro's constant. The mole lets chemists count particles by weighing substances or measuring gas volumes.

The mass of one mole of a substance, measured in grams per mole, numerically equal to its relative molecular or formula mass. It converts mass to moles using moles = mass ÷ molar mass.

An equation in which the number of atoms of each element is the same on both sides. It shows reactants, products and their mole ratios, and state symbols may show solid, liquid, aqueous or gas.

A substance that releases hydrogen ions (H⁺) when dissolved in water, turning blue litmus red and reacting with metals and carbonates. Acids are proton donors.

A substance that accepts hydrogen ions or releases hydroxide ions in water. Bases neutralise acids to form a salt and water; soluble bases are called alkalis.

A compound formed when the hydrogen ion of an acid is replaced by a metal ion or an ammonium ion, for example sodium chloride from hydrochloric acid and sodium hydroxide.

A scale from 0 to 14 that measures how acidic or alkaline a solution is, based on hydrogen ion concentration: below 7 is acidic, 7 is neutral and above 7 is alkaline.

A technique in which a solution of known concentration is added slowly from a burette to a measured volume of another solution until the end point, usually detected with an acid-base indicator.

The change in the amount of reactant used or product formed per unit time. It can be measured by gas volume, mass loss, colour change or the time taken for a cross to disappear.

The idea that particles must collide with enough energy and the correct orientation before a reaction can occur. More frequent or more energetic collisions raise the rate of reaction.

The minimum energy that colliding particles must have before a reaction can happen. A higher activation energy means fewer collisions succeed, so the reaction proceeds more slowly.

A substance that speeds up a reaction by providing an alternative pathway with lower activation energy. It is not used up, and its mass and chemical properties remain unchanged.

The heat energy absorbed or released when a chemical reaction takes place. Exothermic reactions release heat to the surroundings, while endothermic reactions absorb heat from them.

A reaction in which oxidation and reduction occur at the same time. Oxidation is loss of electrons, gain of oxygen or an increase in oxidation number; reduction is the opposite.

A number showing the charge an atom would carry if its compound were fully ionic, for example +2 in MgO. It helps identify which species is oxidised and which is reduced.

A compound containing only carbon and hydrogen, such as methane or ethene. Hydrocarbons are the main constituents of petroleum and natural gas and burn to give carbon dioxide and water.

An atom or group of atoms that gives an organic compound its characteristic reactions, for example the –OH group in alcohols and the –COOH group in carboxylic acids.

A family of organic compounds sharing the same functional group, similar chemical properties and a general formula, in which each member differs from the next by one CH₂ unit.

The industrial route to sulphuric acid: sulphur is burned to sulphur dioxide, which is oxidised to sulphur trioxide over a vanadium(V) oxide catalyst before being absorbed in concentrated sulphuric acid.