Table 1: Hydrogen properties.

Atomic weight 1.0079 g mol−1
Van der Waals radius 120 pm
Covalent radius 31 pm
ΔH (H2 2 H) 436 kJ mol−1
Density at 273 K and 0.1 MPa 0.0899 g l−1
Liquid density (20.25 K) 70.8 g l−1
Boiling point 20.25 K
Critical temperature 32.97 K
Critical pressure 1.276 MPa
Self-ignition temperature 747 K
Flammability limit in air 4.1–74.2 Vol.%
Explosive limits in air 18.3–59Vol.%
Diffusion coefficient in air 0.634 cm2 s−1