(2) H2 Explanation: Hydrogen is a chemical element with symbol H and atomic number 1. With an average atomic weight of 1.00794 u (1.007825 u for hydrogen-1), hydrogen is the lightest element and its monatomic form (H1) is the most abundant chemical substance, constituting roughly 75% of the Universe's baryonic mass. Naturally occurring atomic hydrogen is rare on Earth because hydrogen readily forms covalent compounds with most elements and is present in the-water molecule and in most organic compounds.