Hydrogen is the first element in the periodic table. It has just one electron and has an electronic configuration of 1s1 in the ground state. The atomic number Z of elemental hydrogen is 1 and its atomic mass is also 1.
Hydrogen atom is unique because it does not have any neutrons inside its nucleus. Therefore, all the mass of hydrogen atom is solely due to the mass of protons.
Hydrogen exists in three different isotopes in nature. These are Protium, the most abundant, Dueterium (Heavy Hydrogen) and Tritium (radioactive isotope).
Of these, protium, the most abundant isotope of H alone does not have neutrons in its nucleus. The other two isotopes, Deuterium and Tritium, have one and two neutrons respectively.
Thus, for H atoms
Ground state electronic configuration = 1s1
Hydrogen is extremely reactive in elemental form and thereby combines with another atom of Hydrogen to form molecular Hydrogen, H2,which is also termed as "dihydrogen".
Discovery of Hydrogen: Hydrogen was first discovered by Henry Cavendish while performing the experiment of reacting iron fillings with dil. H2SO4.
The name Hydrogen was suggested by the French chemist Lavoisier. The word, Hydro in Greek means "producer". It is so named, as hydrogen produces water on reacting with oxygen.
Dihydrogen exists in two allotropic forms as Ortho and Para Hydrogens designated as o-H2 and p-H2 respectively.