Helium

Helium is that the element with the symbol He and no 2. With a typical relative mass of 4.002602. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas. It has the lowest boiling and melting point amongst all other elements. Helium is present at about 24% of the total elemental mass. This amount is more than 12 times of all the heavier elements combined. 

The first evidence of helium was observed on August 18, 1868. It was observed as a bright yellow line with a wavelength of 587.49 nanometers in the spectrum of the chromosphere of the Sun. The line was detected by French astronomer Jules Janssen during a total solar eclipse in Guntur, India.

Physical properties
Appearance	A colorless gas, exhibiting a gray, cloudy glow when placed in an electric field or reddish-orange if an especially high voltage is used
Atomic weight	4.003
Atomic number	2
Group	Group 18: Noble gases
Period	Period 1
Block	s-block*
Electron configuration*	1s2
Electrons per shell	2
Phase at STP(Standard Temperature and Pressure)	Gas
Melting point	0.95 K, −272.20 °C, ​−457.96 °F (at 2.5 MPa)
Boiling point	4.222 K, −268.928 °C, ​−452.070 °F
Density at STP when liquid (at m.p.) when liquid (at b.p.)	0.1786 g/L 0.145 g/cm3 0.125 g/cm3
Triple point*	2.177 K, ​5.043 kPa
Critical point*	5.1953 K, 0.22746 MPa
The heat of fusion (H2)	0.0138 kJ/mol
Heat of vaporization	0.0829 kJ/mol
Molar heat capacity	20.78 J/(mol·K)
Vapor pressure	100 Pa: 1.23 K 1000 Pa: 1.67 K 10000 Pa: 2.48 K 100000 Pa: 4.21 K

Atomic properties
Oxidation states*	0
Electronegativity*	Pauling scale- No data
Ionization energies*	1st: 2372.3 kJ/mol 2nd: 5250.5 kJ/mol
Covalent radius*	28 pm
Van der Waals radius*	140 pm

 

1.  A block of the periodic table is a set of elements unified by the orbitals their valence electrons or vacancies lie in.
2.  The electronic configuration is the arrangement of electrons in energy levels around an atomic nucleus.
3.  The triple point of a substance is the temperature and pressure at which the three phases of that substance coexist in thermodynamic equilibrium.
4.  A critical point is the endpoint of a phase equilibrium curve.
5.  The oxidation state sometimes referred to as oxidation number, describes the degree of oxidation of an atom in a chemical compound.
6.  Electronegativity is the tendency of an atom to attract shared electrons to itself.
7.  In physics and chemistry, ionization energy is the minimum amount of energy required to remove the most loosely bound electron of an isolated neutral gaseous atom or molecule.
8.  The covalent radius is a measure of the size of an atom that forms part of one covalent bond.
9.  The Van der Waals radius of an atom is the radius of an imaginary hard-sphere representing the distance of closest approach for another atom.

Source: Wikipedia, Brittanica