Question

Which rule is violated by the electron configuration \[1{s^0}2{s^2}2{p^4}\] and how?

Answer 1

Hint :According to the Aufbau principle electrons fill lower-energy atomic orbitals before filling higher-energy ones. The word Aufbau is from German and it means "building-up". By following the Aufbau principle, we can predict the electron configurations for atoms or ions. The order of filling up in any atom will be \[1s,{\text{ }}2s,{\text{ }}2p,{\text{ }}3s,{\text{ }}3p,{\text{ }}4s,{\text{ }}3d,{\text{ }}4p,{\text{ }}5s,{\text{ }}4d,{\text{ }}5p,{\text{ }}6s,{\text{ }}4f,{\text{ }}5d,{\text{ }}6p,{\text{ }}7s,{\text{ }}5f,{\text{ }}6d,{\text{ }}7p....\]

Complete Step By Step Answer:
In the given question Aufbau principle is violated. According to the rule the lower energy orbitals have to be completely filled first before moving onto higher energy orbitals. Here \[1s\] orbital is having lower energy than \[2s\] orbital but \[1s\] is not filled whereas \[2s\] is. This completely violates the rule.
As a general rule the orbital whose n+l value is the least will have lower energy.
 n + l value of \[1s\] is
\[ \Rightarrow 1{\text{ }} + {\text{ }}0{\text{ }} = {\text{ }}1\]
n + l value of \[2s\] is
\[ \Rightarrow 2{\text{ }} + {\text{ }}0{\text{ }} = {\text{ }}2\]
This means that lower energy is for the \[1s\] orbital since it has lower value of n+l
But we see that in this question electrons are filled in \[2s\] orbital without filling \[1s\] orbital which has lower n + l value.. Again we proved that the Aufbau principle is violated in the given electronic configuration.

Note :
There are certain cases in which the Aufbau rule is violated. This may be due to increased stability reasons. An example is Chromium. It has an electronic configuration of \[4{s^1}3{d^5}\]. This is allowed because by attaining this electronic configuration chromium becomes more stable. The reason for that is half-filled d and s orbital. There are other elements such as copper, silver and gold which also violate the Aufbau principle for more stability.