Hydrogen (H)  Symbol	H Atomic Number	1 Atomic Weight	1.00794 Group	1 (alkali metals or hydrogen family/lithium family) IA (Old IUPAC), IA (CAS) Period	1 Block	s     Planetary Bohr Model of H  Lewis Dot Diagram of H Physical Properties Density	0.0000899 g/cm3 Atomic Volume	14.1 cm3/mole Atomic Radius	2.08 angstroms Covalent Radius	37 pm Radioactive	No  States Melting Point	-259.1 °C Boiling Point	-252.9 °C Heat Of Fusion	0.05868 kJ/mole Heat Of Vaporization	0.4581 kJ/mole  Energy Electronegativity	2.2 (Pauling Scale) Electron Affinity	-73 kJ/mol 1st Ionization Energy	1312 kJ/mol Specific Heat Capacity	14.304 J g−1 K−1 Thermal Conductivity	0.1815 W·m−1·K−1  Electrons and Oxidation Oxidation States	±1 Electrons Per Shell	1 Electron Configuration	1s1   Helium (He)  Symbol	He Atomic Number	2 Atomic Weight	4.002602 Group	18 (noble gases or helium family/neon family) VIII (Old IUPAC), VIIIA (CAS) Period	1 Block	s     Planetary Bohr Model of He  Lewis Dot Diagram of He Physical Properties Density	0.0001785 g/cm3 Atomic Volume	31.8 cm3/mole Atomic Radius	 angstroms Covalent Radius	32 pm Radioactive	No  States Melting Point	-272.2 °C Boiling Point	-268.9 °C Heat Of Fusion	0.021 kJ/mole Heat Of Vaporization	0.0845 kJ/mole  Energy Electronegativity	 (Pauling Scale) Electron Affinity	 kJ/mol 1st Ionization Energy	2377 kJ/mol Specific Heat Capacity	5.193 J g−1 K−1 Thermal Conductivity	0.152 W·m−1·K−1  Electrons and Oxidation Oxidation States	0 Electrons Per Shell	2 Electron Configuration	1s2