Key Definitions

Atom

The smallest particle of a chemical element that can exist

Element

A substance consisting of atoms which all have the same number of protons - i.e. the same atomic number

Proton

A stable subatomic particle occurring in all atomic nuclei, with a positive electric charge equal in magnitude to that of an electron

Neutron

A subatomic particle of about the same mass as a proton but without an electric charge, present in all atomic nuclei except those of ordinary hydrogen

Electron

A stable subatomic particle with a charge of negative electricity, has a relative mass 1/2000th that of a proton

Nucleus

The positively charged central core of an atom, consisting of protons and neutrons and containing nearly all its mass

Atomic Number

The number of protons in the nucleus of an atom, which is characteristic of a chemical element and determines its place in the periodic table. Also known as an element’s proton number

Isotope

Forms of the same element with the same number of protons, but different number of neutrons

Mass Number

The total number of protons and neutrons in a nucleus. Different isotopes of the same element have different mass numbers

Relative Atomic Mass (Ar)

The ratio of the average mean mass of one atom of an element to one twelfth of the mass of an atom of carbon-12. It is an average of the mass numbers of all the different isotopes of that element