How do you determine the molecular formula/chemical formula of a single element?

Question

For example, Hydrogen is H₂ and Oxygen is O_2. How did we get the 2 subscripts for each?

Thank you so much

Answer 1

While the explanation provided by Arturo is correct, I thought you might want a further, more detailed explanation as to WHY they exist as diatomic molecules.  Taking O₂ as an example:  Each O by itself has 6 valence electrons, but it wants 8 to be stable (see octet rule).  So, 2 oxygen atoms combine with a double bond, leaving each O with 2 lone pairs of electrons, and each O with 8 electrons.  (Look up the Lewis dot structure of O₂ to see a diagram).

 

 

Answer 2

Under natural conditions, hydrogen gas and oxygen gas exist as diatomic molecules, and hence the formulas H₂ and O₂.