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Polyatomic elements are those that require two or more atoms bonded into a molecule to exist naturally.

The polyatomic elements we currently know of are:

Astatine
At2

Bromine
Br2

Chlorine
Cl2

Fluorine
F2

Hydrogen
H2

Iodine
I2

Nitrogen
N2

Oxygen
O2/O3
(O3 is ozone)

Phosphorus
P4

Sulphur
S8

Note: The ones listed with a subscript of 2 are technically diatomic (or biatomic) elements, but I decided to include them here anyway.