What do 4 hydrogen and 2 oxygen atoms chemically react to

2 molecules of water

Explanation:

In chemical reactions, molecules and compounds chemically combine to give products. This involves atoms combining by re-establishing bonding relationships.

  4 hydrogen atoms is equivalent to 2 molecules of Hydrogen gas = H₂

   2 oxygen atoms is the same as 1 oxygen molecule  = O₂

This 2 molecules of hydrogen gas and 1 molecule of O₂ gas will combine to form 2 molecules of water chemically;

                        2H₂   + O₂    →  2H₂O