 The universal emergence of atomic hydrogen first occurred during the recombination epoch . At standard temperature and pressure , hydrogen is a colorless , odorless , tasteless , non @-@ toxic , nonmetallic , highly combustible diatomic gas with the molecular formula H2 . Since hydrogen readily forms covalent compounds with most non @-@ metallic elements , most of the hydrogen on Earth exists in molecular forms such as water or organic compounds . Hydrogen plays a particularly important role in acid – base reactions because most acid @-@ base reactions involve the exchange of protons between soluble molecules . In ionic compounds , hydrogen can take the form of a negative charge ( i.e. , anion ) when it is known as a hydride , or as a positively charged ( i.e. , cation ) species denoted by the symbol H + . The hydrogen cation is written as though composed of a bare proton , but in reality , hydrogen cations in ionic compounds are always more complex . As the only neutral atom for which the Schrödinger equation can be solved analytically , study of the energetics and bonding of the hydrogen atom has played a key role in the development of quantum mechanics . 
