Question

How do you write the noble-gas electron configuration for Helium?

Answer 1

Electron configuration of an atom tells us how the electrons are distributed in different energy shells, subshells and orbitals of the atoms. Electrons are filled in increasing order of energy of these shells/subshells/orbitals. The increasing order of energy of subshells and thus order of filling of electrons is following:
$1s2s2p3s3p4s3d4p5s....................................$

Complete solution:
Atomic number of helium: 2
So, the electrons will be filled as following:
$1{s^2}$
We know that the maximum number of electrons that any orbital can accommodate is 2 in accordance with Pauli’s Exclusion principle.
This is the complete electron configuration of helium. To write noble gas electron configuration of any element, we have to write the electron configuration of the noble gas coming before that element.
But we know that helium itself is a noble gas having electronic configuration of $1{s^2}$
To write noble gas electron configuration of helium, we just have to write $\left[ {He} \right]$ in place of $1{s^2}$
Now, the noble gas electron configuration of He becomes $\left[ {He} \right]$
So, $\left[ {He} \right]$ is the required noble gas electron configuration for Helium.

Additional information: Energy shells consist of different subshells such as s, p, d and f subshells. Each subshell has a different number of orbitals. For example, s subshells have only 1 orbital. Similarly, p, d and f subshells have 3,5 and 7 orbitals respectively. Maximum 2 electrons can be filled in one orbital. Thus, s subshell can have maximum 2 electrons (because it has only one orbital). Similarly, p, d and f subshells can have 6, 10 and 14 electrons respectively.

Note: It has to be noted that a noble gas configuration of an atom consists of the elemental symbol of the last noble gas prior to that atom, followed by the configuration of other remaining electrons. Also, this needs to be made clear that the helium 1s orbital is full with 2 electrons and any additional electrons would go in a new energy level. The electronic configuration for helium shows a full noble gas configuration.