Name
Hydrogen
Helium
Symbol
H
He
Atomic Number
1
2
Group
1
18
Period
1
1
Block
s
s
Abundance in Earth's crust
0.15%
5.5×10-7%
Abundance in Universe
75%
23%
Atomic Weight
1.00794
4.002602
Category
Other nonmetals
Noble gases
Color
Colorless
Colorless
Radioactive
No
No
Crystal Structure
Simple Hexagonal
Face Centered Cubic
Density (g/cm3)
0.00008988
0.0001785
Phase
Gas
Gas
Melting Point
14.01 K | -259.14 °C | -434.45 °F
0.95 K | -272.2 °C | -457.96 °F
Boiling Point
20.28 K | -252.87 °C | -423.17 °F
4.22 K | -268.93 °C | -452.07 °F
Heat of Fusion (kJ/mol)
0.558
0.02
Heat of Vaporization (kJ/mol)
0.452
0.083
Specific Heat Capacity (J/g·K)
14.304
5.193
Atomic Radius
53 pm
31 pm
Covalent Radius
31 pm
28 pm
Electronegativity (Pauling scale)
2.2
-
Ionization Potential (eV)
13.5984
24.5874
Atomic Volume (cm3/mol)
14.4
27.2
Thermal Conductivity (W/cm·K)
0.001815
0.00152
Oxidation States
-1, 1
0
Electron Configuration
1s1
1s2
Electrons per shell
1
2