I believe it's the second option. 2 or more elements joined together such that the elements have lost their individual identity in favour of a new set of properties.
H2O means water, which is made of two hydrogen and one oxygen atoms.
Oxygen is a gas that your body needs to function. Oxygen comes from the air.
2Al + 3Cl₂ → 2AlCl₃
mol Al = 2/3 x 1.25 = 0.83
mass Al = 0.83 x 27 g/mol = 22.41 g
The trough and the hill part
