Question

Write the electronic configuration of the following of the following element:

${}_{15}{}^{31}\mathrm{P}$

Answer 1

Electronic configuration: It is the distribution of electrons in the various energy levels or orbits or shells around the nucleus of an atom.

Step-1: Find out the total number of electrons in the atom

In Phosphorous (P), ${}_{15}{}^{31}\mathrm{P}$

As for any element X, atomic number and mass number is written as ${}_{\mathrm{Atomic}\mathrm{number}}{}^{\mathrm{Mass}\mathrm{number}}\mathrm{X}$

• Therefore, for Phosphorous, Atomic number= 15
• Also, Atomic number= Number of protons
• So, number of protons= 15
• Since an atom is electrically neutral, therefore number of protons= number of electrons
• Therefore, number of electrons= 15

Step-2: Follow Bohr-Bury scheme to write electronic configuration

1. Distribute the electrons from shell 1 or shell K onwards, such as 1, 2, 3….etc. or K,L, M ……etc.
2. Fill the shell upto its maximum capacity and then move to the next shell. Maximum capacity of a shell is counted from the formula 2n² where n= 1 for shell 1 (or K) , 2 for shell 2 (or L) and so on.
3. Maximum number of electrons in valence shell is 8.

Shell	Maximum capacity	Electrons added
K	2	2
L	8	8
M	18	5

Here 5 electrons are added to M shell because only 15 electrons need to be distributed. As 2+ 8+ 5 makes a total of 15, stop after 15 electrons are distributed.

So the electronic configuration of ${}_{15}{}^{31}\mathrm{P}$ in K, L and M shell is 2, 8, 5 respectively.