In the balanced chemical reaction \( 2H_2 + O_2 \rightarrow 2H_2O \), there are four atoms of hydrogen (H) before the reaction because each \( H_2 \) molecule contains 2 hydrogen atoms, and there are 2 \( H_2 \) molecules (2 x 2 = 4).
After the reaction, the products are \( 2H_2O \). Each water molecule (H2O) contains 2 hydrogen atoms, and there are 2 water molecules produced (2 x 2 = 4).
Thus, there are also 4 atoms of hydrogen present after the reaction.
The correct answer is 4.