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All matter in the universe is made up of different particles. In chemistry, there are two main types of particle that we need to know about.

Atom

• This is the smallest particle of an element.
• Scientists originally thought that it was impossible to split the atom, but this is not the case.

Molecule

• A cluster of non-metal atoms that are chemically bonded together.
• The atoms in a molecule are joined by covalent bonds.
• The atoms always join in fixed ratios and molecules have a specific formula, eg H₂O or N₂.
• There are molecules of compounds (eg CH₄) and molecules of elements (eg O₂).

When atoms and molecules lose or gain electrons, they form ions.

Ion

An ion is an electrically charged particle, formed when an atom or a molecule loses or gains electrons.

• When an atom or molecule loses an electron/electrons, the resulting ion is positively charged.
• When atom or molecule gains an electron/electrons, the resulting ion is negatively charged.
• The size of the charge on the ion is equal to the number of electrons lost or gained.