TASC Chemistry Level 4 key terms

The smallest particle of an element that still keeps the chemical properties of that element. It has a tiny nucleus of protons and neutrons, surrounded by electrons.

A group of two or more atoms held together by covalent bonds. It acts as a single unit and may contain atoms of the same element or of different elements.

An atom or group of atoms that has gained or lost electrons, giving it a net electrical charge. Positive ions are cations; negative ions are anions.

One of two or more forms of the same element whose atoms have equal numbers of protons but different numbers of neutrons, so they differ in mass but behave alike chemically.

The amount of substance that contains as many particles as there are atoms in 12 g of carbon-12, which is about 6.02 x 10^23 particles.

The mass of one mole of a substance, found by adding together the relative atomic masses of every atom shown in its formula. Units are grams per mole.

The simplest whole-number ratio of the atoms of each element in a compound. For example, hydrogen peroxide has the empirical formula HO.

A chemical bond formed when two atoms share one or more pairs of electrons. It typically joins non-metal atoms and may be single, double or triple.

The strong attraction between oppositely charged ions that holds an ionic lattice together. It forms when a metal atom transfers electrons to a non-metal atom.

An attraction between a hydrogen atom that is covalently bonded to an electronegative atom and a lone pair of electrons on another electronegative atom nearby.

A measure of how strongly an atom pulls the shared pair of electrons in a covalent bond towards itself. It rises across a period and falls down a group.

A change in which a substance loses electrons, gains oxygen or loses hydrogen. It cannot happen alone, because reduction always occurs at the same time.

A reaction in which electrons are transferred between species, so oxidation and reduction happen together. Combustion, corrosion and displacement reactions are typical examples.

A substance that can donate a proton (hydrogen ion) to another substance. In water it raises the hydrogen ion concentration and gives a pH below 7.

A number, usually between 0 and 14, that shows how acidic or alkaline a solution is by measuring its hydrogen ion concentration.

The minimum energy that colliding particles must have for a reaction to happen. It is a barrier, so reactions with a high value are slow unless conditions change.

A substance that increases the rate of a reaction by offering a route with lower activation energy. It is not used up and can be recovered chemically unchanged.

The state of a reversible reaction in a closed system when the forward and reverse reactions occur at equal rates, so the amounts of reactants and products stay constant.

If a system at equilibrium is disturbed by a change in concentration, pressure or temperature, the system shifts in the direction that opposes that change.

The heat energy transferred to or from the surroundings during a reaction at constant pressure. It is negative for exothermic reactions and positive for endothermic ones.

The reactant that is used up first in a reaction. Its amount decides the greatest possible mass of product, so the other reactants are said to be in excess.

The amount of solute present in a stated volume of solution. It is often measured in moles per cubic decimetre or in grams per litre.

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.

The point during a titration when the indicator changes colour or the measured signal alters, showing that the addition should stop. It should be very close to the equivalence point.

A solution whose concentration is known to a high degree of accuracy, usually made by dissolving an exactly known mass of a pure substance in a volumetric flask.

The mass of product actually made, divided by the maximum mass predicted by the balanced equation, then multiplied by 100 to give a percentage.

How close a measured result is to the true or accepted value. Poor accuracy is usually caused by systematic errors that shift all readings the same way.

How closely repeated measurements agree with one another. Results can be precise but still inaccurate if a systematic error affects them all.

A factor that is deliberately kept constant across all trials of an experiment so that it cannot affect the result being measured.

The degree to which an experiment really measures the quantity it claims to measure, which depends on controlling variables and choosing a suitable method.

A chemical equation in which the number of atoms of each element is the same on both sides, showing that matter is conserved during the reaction.

A letter in brackets written after a formula in a chemical equation to show the physical state: (s) solid, (l) liquid, (g) gas or (aq) aqueous.

The internationally agreed set of rules for naming chemical compounds, so that any structure can be described by a single unambiguous name and vice versa.

An atmospheric gas that absorbs and re-emits infrared radiation, slowing the escape of heat into space. Carbon dioxide, methane and water vapour are examples.

A natural material or energy source that is replaced by natural processes as fast as it is used, such as sunlight, wind or timber from managed forests.

A method of estimating the total environmental impact of a product from extraction of raw materials through manufacture, transport, use and final disposal.

The build-up of a substance, such as a persistent pollutant, inside the tissues of living organisms over time, because it is taken in faster than it is removed.

The percentage of the total mass of reactants that ends up in the desired product. A high value means the reaction wastes few atoms.