Question

What is the electron configuration for potassium?
(A) $1s^{2}2s^{2}2p^{6}3s^{2}3p^5$
(B) [Ar] $4s^1$
(C) $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^{2}3d^5$
(D) $3s^{2}4s^{2}4p^1$

Answer 1

Hint: In order to answer this question you need to recall the concept of Aufbau principle and Hund’s rule. The word ‘aufbau’ in German means ‘building up’. Here it refers to the filling up of the orbital with electrons.

Complete step by step answer:
The Aufbau principle states that in the ground state of an atom, an electron enters the orbital with lowest energy first and the subsequent electrons are filled in the order of increasing energies. In simple words, it means filling of electrons from lower to higher energy.
The Aufbau principle determines how electrons organize into shells and subshells around the atomic nucleus.
- Electrons go into the subshells having the lowest possible energy.
- An orbital can hold at a maximum of 2 electrons obeying the Pauli Exclusion Principle.
- Electrons obey Hund’s rule, which states that electrons arrange in a way that the electron spin is maximum if degenerate orbitals are available.
- Potassium atoms have 19 electrons in its structure.
- The ground state electronic configuration of K is $1s^{2}2s^{2}2p^{6}3s^{2}3p^{6}4s^1$.
- Now, electronic configuration of the noble gas Ar is $1s^{2}2s^{2}2p^{6}3s^{2}3p^6$. So, we can write the ground state electron configuration of K as $[Ar]4s^1$. As we know potassium is a smaller atom, i.e. have a small size. Therefore, it has a greater effective nuclear charge.
- Hence we can say that the electronic configuration of potassium is $[Ar]4s^1$.
So, the correct answer is “Option B”.

Note: Note that 4s orbital has lower energy than 3d orbital and hence according to Aufbau’s principle, the electrons are first filled in 4s orbital even though its primary quantum number is higher than 3d orbital.