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A chemical formula is used to represent an element or compound in balanced chemical equations. Formulae differ, depending on the elements they represent.

The formulae for most elements is just their symbol, but there is a group of non-metal elements that exist as molecules that contain two atoms joined together. To show this, their formulae all have a subscript number 2 in them. For example:

• iodine, I₂
• bromine, Br₂
• chlorine, Cl₂
• fluorine, F₂
• oxygen, O₂
• nitrogen, N₂
• hydrogen, H₂

A mnemonic can be used to remember these elements:

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