Question

Write the electronic configuration of the following element:

${}_{19}{}^{39}\mathrm{K}$

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 species

In Potassium (K), ${}_{19}{}^{39}\mathrm{K}$

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 Potassium, Atomic number = 19
• Also, Atomic number = Number of protons
• So, number of protons = 19
• Since an atom is electrically neutral, therefore the number of protons= number of electrons
• Therefore, number of electrons = 19

Step-2: Follow the Bohr-Bury scheme to write an 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	8 (exception)
N	32	1

Exception:

• In case of Potassium, note that M shell is expected to accommodate 9 electrons as its maximum capacity is of 18 electrons. But this does not happen.
• The reason is that electrons are filled in the orbits in the order of their increasing energy levels i.e.; the orbit with lower energy is filled first and the one with higher energy is filled afterwards.
• After 8 electrons are filled in M shell, the energy of N shell becomes lower than the M shell. As mentioned above, the lower energy orbit is filled first due to which the 9th electron occupies N shell making the configuration as 2, 8, 8, 1 instead of 2, 8, 9.

As 2+ 8+ 8+ 1 makes a total of 19, stop after 19 electrons are distributed.

So the electronic configuration of ${}_{19}{}^{39}\mathrm{K}$ in K, L, M and N shell is 2, 8, 8, 1 respectively.