Question

Which rule is violated by the electron configuration $$1s^{0}2s^{2}2p^4$$ 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 $$1s,2s,2p,3s,3p,4s,3d,4p,5s,4d,5p,6s,4f,5d,6p,7s,5f,6d,7p....$$

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
$$\Rightarrow 1+0=1$$
n + l value of $$2s$$ is
$$\Rightarrow 2+0=2$$
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 $$4s^{1}3d^5$$. 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.