Question

If K and L shells of an atom are full, then what would be the total number of electrons in the atom? What is the valency of this element?

Answer 1

Part-1

According to Bohr’s Atomic model electrons revolve around the nucleus in a specific circular path known as orbit or called the shell. Shells have stationary energy levels, the energy of each shell is constant. These shells are called energy levels. It is numbered as $1,2,3,4.....$or $\mathrm{K},\mathrm{L},\mathrm{M},\mathrm{N},.....$ from the nucleus outwards.

The value of ‘n’	Designation of shell
$1$	$\mathrm{K}$
$2$	$\mathrm{L}$
$3$	$\mathrm{M}$
$4$	$\mathrm{N}$
$5$	$\mathrm{O}$

The maximum number of electrons present in a shell is given by the formula $2{\mathrm{n}}^2$, where ‘$\mathrm{n}$’ is represents the principal quantum number. It represents the size and the energy of the shell.

Hence, the maximum number of electrons in different shells are as follows:

1. First orbit or $\mathrm{K}$-shell ($\mathrm{n}=1$) =
$2\times 1^2$$=2$
2. Second orbit or $\mathrm{L}$-shell ($\mathrm{n}=2$) =
$2\times 2^2=8$
and so on.

Therefore, the K shell can accommodate $\mathbf{2}$ electrons whereas L can accommodate $\mathbf{8}$ electrons, therefore total electrons are$\mathbf{10}$.

Part-2

Valency is simply equal to the number of electrons gained, lost or shared by an atom of an element to achieve the nearest noble gas configuration. As $\mathrm{K}$ and $\mathrm{L}$ are fully filled, so it will gain or lose electrons, therefore valency is zero.