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Elements which exist as Molecules.

Hydrogen, nitrogen, oxygen and the halogens
exist as diatomic molecules (molecules containing two atoms).
See atomic structure for pictures of these molecules.

One mole of hydrogen (H2) molecules
has a mass of 2 x the relative atomic mass (RAM) of hydrogen.
This is because each molecule of hydrogen contains two atoms
and the "relative atomic mass" is the relative mass of one atom.
So, one mole of hydrogen molecules has a mass of 2 g (2 x 1).
This is called the relative molecular mass (RMM).

One mole of nitrogen (N2) molecules (RAM = 14)
has a mass of 2 x 14 grams,  = 28 g. RMM = 28.

One mole of oxygen (O2) molecules (RAM = 16)
has a mass of 2 x 16 grams,  = 32 g. RMM = 32.

One mole of chlorine (Cl2) molecules (RAM = 35.5)
has a mass of 2 x 35.5 grams,  = 71 g. RMM = 71.

One mole of bromine (Br2) molecules (RAM = 80)
has a mass of 2 x 80 grams,  = 160 g. RMM = 160.

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