In the chemical reaction \(2H_2 + O_2 \rightarrow 2H_2O\), there are 4 atoms of hydrogen (since \(2H_2\) means 2 molecules of hydrogen, and each molecule contains 2 hydrogen atoms).
After the reaction, the product is \(2H_2O\), which contains 2 molecules of water. Each water molecule (H₂O) contains 2 hydrogen atoms, so \(2H_2O\) contains a total of \(2 \times 2 = 4\) hydrogen atoms.
Therefore, the number of hydrogen atoms present after the reaction is 4.