2 2

Helium, He, element 2

Noble gas· Period 1· Group 18· s-block

Configuration 1s2

Atomic weight
4.0026Da
Phase
gas
Density
1.64×10−4g/cm³
Melting point
0.95K -272.2 °C
Boiling point
4.222K -268.93 °C
Electronegativity
No data

Colourless, odourless gaseous nonmetallic element. Belongs to group 18 of the periodic table. Lowest boiling point of all elements and can only be solidified under pressure. Chemically inert, no known compounds. Discovered in the solar spectrum in 1868 by Lockyer. Description from mendeleev.

Back to the table
The outermost shell: the valence electronsThe inner shellsThe nucleus One bead for each electron, on the ring of its shell, counted from the nucleus out. The rings turn at their own pace; the model is Bohr's picture of the atom, which places electrons in shells but not on true orbits.

Atomic properties

5 of 5 fields have a value for helium

Atomic weight The standard atomic weight. For an element with no stable isotope, the mass of its longest-lived isotope.
4.002602Da
Source: mendeleev
Electron configuration
1s2
Source: mendeleev
Uncertainty of the atomic weight
2×10−6Da
Source: mendeleev
Electron configuration in full The electron configuration with the noble-gas core written out.
1s2
Source: Worked outfrom electron configuration
Electrons in each shell Counted from the electron configuration, from the first shell outward.
2
Source: Worked outfrom electron configuration

Screening of each subshell

Clementi's screening constants: how much of the nucleus's charge the other electrons hide from an electron in each subshell. What is left is the effective nuclear charge it feels.

SubshellScreening σEffective charge Z − σ
1s0.31251.6875