2025 IGCSE - Atoms, Elements and Compounds

Element

Made up of ONE type of atom only

Compound

Made up of two or more different types of atoms that are chemically bonded

Mixture

Made up of two or more types of atoms that are NOT chemically bonded

Outer Electron Shell

Holds electrons

Nucleus

Contains protons and neutrons

A proton…

has a positive charge and a relative mass of 1.

A neutron…

has a neutral (no) charge and a relative mass of 1.

An electron…

has a negative charge and a relative mass of 1/1836.

Atomic Number

Number of protons in the the nucleus of an atom.

Mass Number

Total number of protons AND neutrons in an atom.

Group VIII noble gasses have…

a full outer shell.

The number of outer-shell electrons is equal to the group number for groups

I to VII

Number of occupied electron shells =

period number

Positive ions, or cations, are formed when…

an atom LOSES one or more electrons.

Negative ions, or anions, are formed when…

an atom GAINS one or more electrons.

An ionic bond is a …

strong electrostatic attraction between oppositely charged atoms.

Group I are…

alkali metals.

Group VII are…

halogens.

A metal generally forms a

positive ion (cation).

A non-metal generally forms a

negative ion (anion).

Properties of Ionic Compounds

High melting and boiling points, good electrical conductivity when molten or aqueous, generally soluble in water

Ionic compounds have…

a lattice structure with alternating positive and negative ions.

Covalent Bond

Formed when a pair of electrons is shared between two atoms.

Properties of Simple Molecular Compounds

Low melting and boiling points, poor electrical conductivity.