Question

What is the electron configuration for an excited atom of phosphorus, ready to emit a photon of energy and return to ground state? (Configuration has to follow all rules except be in lowest energy).

Answer 1

Hint: We know that the ground state of the atom is the state in which all electrons in a system are present in the lowest possible energy levels whereas in the excited state, an electron jumps from lower energy level to higher energy level and the system has overall higher energy than ground state.

Complete Step By Step Answer:
To find the electronic configuration for an excited atom of phosphorus, we must first write its electronic configuration at ground state. The electronic configuration of any atom obeys three basic principles i.e., Aufbau principle, Pauli exclusion principle and Hund’s rule.
Aufbau principle: It states that in the ground state of an atom, the electrons are first filled in lower energy orbitals before occupying the orbitals of higher energy.
Pauli exclusion principle: It states that in a single atom, no two electrons can have the same set of quantum numbers. In other words, only two electrons can occupy a single orbital and the spin of the electrons must be opposite.
Hund’s rule of maximum multiplicity: It states that every orbital in a subshell is singly occupied before the pairing of electrons.
So, the electronic configuration of phosphorus $(Z=15)$ at the ground state will be as follows:
 ${}_{15}P=1s^{2}2s^{2}2p^{6}3s^{2}3p^{3}3d^0$
Now, at excited state of the atom, an electron from 3s subshell will jump to the orbital of 3d vacant subshell and the electronic configuration of the element will be as follows:
 ${}_{15}P=1s^{2}2s^{2}2p^{6}3s^{1}3p^{3}3d^1$
The electron in its excited state is unstable and tends to rearrange itself in order to return to its lowest energy state. Thus, the electron emits a photon of the same energy it absorbed to reach its excited state and return back to the ground state.
Hence, the electronic configuration of excited atom of phosphorus is $1s^{2}2s^{2}2p^{6}3s^{1}3p^{3}3d^1$ .

Note:
It is important to note that the electron from 3p orbital will not get excited and reach vacant d orbital because of the two facts, first is the excitation always takes place for a paired electron and second is there are three unpaired electrons present in 3p orbital i.e., it is in its stable half-filled state, so there is a less chance for an electron from 3p orbital to get excited to reach 3d orbital.