#2 Hellium

is a chemical element with symbol H and atomic number 1. With a standard atomic weight of1.008, hydrogen is the lightest element on the periodic table. Its monatomic form (H) is the most abundant chemical substance in the Universe, constituting roughly 75% of all baryonic mass.

Atomic weight: 4.0026

Melting point: none

Boiling point: -268.9 C (-452 F)

Density: (1 atm, 0 C) 0.1785 gram/litre