Chemistry Year 9 2026 Revision

Atom

smallest unit of an element

proton

positively charged particle located in the nucleus

negative ion

atom that has gained electrons/lost protons and has a negative charge

product

A substance formed during a chemical reaction

element

A pure substance made of only one type of atom.

electron

A negatively charged subatomic particle that moves around the nucleus

isotope

Atoms of the same element with different numbers of neutrons

exothermic

A reaction that releases energy (usually heat)

pure element

A substance made of only one type of atom and not mixed with anything else

neutron

A subatomic particle with no charge found in the nucleus

electron shell

A layer around the nucleus where electrons are found

endothermic

A reaction that absorbs energy from its surroundings

molecule

Two or more atoms chemically bonded together

atomic number

The number of protons in an atom's nucleus

valence shell

The outermost electron shell of an atom

chemical equation

A symbolic way of showing a chemical reaction using formulas.

compound

A substance made of two or more different elements chemically bonded together.

atomic mass

The total number of protons and neutrons in an atom

half life

The time it takes for half of a radioactive substance to decay.

word equation

A chemical reaction written using the names of substances instead of formulas.

lattice

A regular 3D arrangement of atoms, ions or molecules

electrically neutral

Equal protons and electrons.

chemical change

New substances are formed.

law of conservation of matter

matter cannot be created or destroyed

mixture

A combination of two or more substances that are not chemically combined

ion

A charged atom

physical change

Change with no new substance

unbalanced

Atoms not equal on both sides

subatomic particles

protons, neutrons, electrons

positive ion

Atom that lost electrons

reactant

Starting substance in a reaction

balanced

Same number of atoms on both sides