HELIUM

hydrogen ← helium → lithium - ↑ He ↓ Ne 2He Periodic table
Appearance
Colorless gas, exhibiting a purple glow when placed in a high voltage electric field Spectral lines of Helium.
General properties
Name, symbol, number	helium, He, 2
Pronunciation	/ˈhiːliəm/ HEE-lee-əm
Element category	noble gases
Group, period, block	18, 1, s
Standard atomic weight	4.002602(2) g·mol−1
Electron configuration	1s2
Electrons per shell	2 (Image)
Physical properties
Phase	gas
Density	(0 °C, 101.325 kPa) 0.1786 g/L
Melting point	(at 2.5 MPa) 0.95 K, −272.20 °C, −457.96 °F
Boiling point	4.22 K, −268.93 °C, −452.07 °F
Critical point	5.19 K, 0.227 MPa
Heat of fusion	0.0138 kJ·mol−1
Heat of vaporization	0.0829 kJ·mol−1
Specific heat capacity	(25 °C) 20.786 J·mol−1·K−1
Vapor pressure (defined by ITS-90)
P/Pa 1 10 100 1 k 10 k 100 k at T/K     1.23 1.67 2.48 4.21
Atomic properties
Electronegativity	no data (Pauling scale)
Ionization energies	1st: 2372.3 kJ·mol−1
	2nd: 5250.5 kJ·mol−1
Covalent radius	28 pm
Van der Waals radius	140 pm
Miscellanea
Crystal structure	hexagonal close-packed
Magnetic ordering	diamagnetic[1]
Thermal conductivity	(300 K) 0.1513 W·m−1·K−1
Speed of sound	972 m/s
CAS registry number	7440-59-7
Most stable isotopes
Main article: Isotopes of helium
iso NA half-life DM DE (MeV) DP 3He 0.000137%* 3He is stable with 1 neutron 4He 99.999863%* 4He is stable with 2 neutrons *Atmospheric value, abundance may differ elsewhere.
v • d • e

Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert monatomic gas that heads the noble gas group in the periodic table. Its boiling and melting points are the lowest among the elements and it exists only as a gas except in extreme conditions. Next to hydrogen, it is the second most abundant element in the universe, and accounts for 24% of the elemental mass of our galaxy.

An unknown yellow spectral line signature in sunlight was first observed from a solar eclipse in 1868 by French astronomer Pierre Janssen. Janssen is jointly credited with the discovery of the element with Norman Lockyer, who observed the same eclipse and was the first to propose that the line was due to a new element which he named helium. In 1903, large reserves of helium were found in the natural gas fields of the United States, which is by far the largest supplier of the gas.