1) Hydrogen is the first element on the periodic table and is denoted by the atomic symbol H. It has an atomic number of 1 and an atomic mass of 1.008 unified atomic mass units.
Hydrogen is a gas at standard temperature and pressure (STP), with a melting point of -259.14 �C and a boiling point of -252.87 �C. It has a mass density of 8.99�10^-5 g/cm^3 and a sound speed of 1270 m/s. Its thermal conductivity is 0.1805 W/(m K).
In terms of electromagnetic properties, hydrogen is diamagnetic and colorless, with a refractive index of 1.000132.
Hydrogen is a reactive element with a valence of 1, an electronegativity of 2.2, an electron affinity of 0.755 eV, and a first ionization energy of 13.598 eV.
The atomic radius of hydrogen in its electronic ground state is 25 picometers.
Hydrogen is the most abundant element in the universe, making up 75% of its mass. It is the 11th most abundant element in the Earth's crust and the 3rd most abundant in the human body.
Hydrogen has two stable isotopes: H-1 (99.985%) and H-2 (0.015%). It also has unstable isotopes, including H-3, which has a half-life of 12.32 years.
The CAS number for hydrogen is 1333-74-0 and its PubChem CID number is CID783.