Question

An element has an atomic number 20. How many electrons will be present in K, L, M and N shells of its atom?
A.2,8,6,4
B.2,8,7,3
C.3,7,7,3
D.2,8,8,2

Answer 1

Hint:There are total four types of quantum numbers i.e.
-Principal quantum number
-Azimuthal quantum number
-Magnetic quantum number
-Spin quantum number.
This question is based on the principal quantum number (n) which gives information about the shell in which electrons are present.

Complete step by step answer:
The element has an atomic number 20.
Hence there electronic configuration is: \[1{s^2}2{s^2}2{p^6}3{s^2}3{p^6}4{s^2}\]
Principal quantum number gives information about the shell in which the electron is present. There are different shells and for specific shells the specific value of principal quantum number. Principal quantum number is denoted by “n”.
For shell Value of principal quantum number (n)
K = 1
L = 2
M = 3
N = 4
O = 5
So from electronic configuration it was observed that
K shell contains total 2 electrons (\[1{s^2}\])
L shell contains total 8 electrons (\[2{s^2}2{p^6}\])
M shell contains total 8 electrons (\[3{s^2}3{p^6}\])
N shell contains total 2 electrons (\[4{s^2}\])
Therefore the correct option is D.

Note:
Quantum number is the set of four numbers which gives complete information about the electron that is its location, energy and orientation of the orbital, etc.
-Principal quantum number gives information about the shell in which the electron is present. it also tells about size of shell and energy of orbital
-Azimuthal or angular quantum number gives information about subshells in which electrons are present and also tells about energies and shape of that subshells. Also gives information about orbital angular momentum.
-Magnetic quantum number gives information about orientation of orbital and number of orbital in a given subshell.
-Spin quantum number tells about the direction of spin of an electron in any orbital.it has two values +1/5 or – 1/5.