Which is not a ground state electron configuration?

A. 1s2
B. 1s2 2p1
C. 1s2 2s2
D. 1s2 2s2 2p1

To determine which electron configuration is not a ground state electron configuration, we need to understand what a ground state is.

The ground state represents the lowest energy level that an electron can occupy within an atom. According to the Aufbau principle, electrons fill orbitals in order of increasing energy. Each orbital can hold a maximum number of electrons, and once an orbital is filled, electrons begin to fill the next available orbital of higher energy.

Let's analyze the given options:

A. 1s2 - This configuration represents the ground state of the helium atom. It has two electrons filling the 1s orbital. This is a valid ground state configuration.

B. 1s2 2p1 - This configuration represents the ground state of the boron atom. It has two electrons filling the 1s orbital and one electron filling the 2p orbital. This is a valid ground state configuration.

C. 1s2 2s2 - This configuration represents the ground state of the beryllium atom. It has two electrons filling the 1s orbital and two electrons filling the 2s orbital. This is a valid ground state configuration.

D. 1s2 2s2 2p1 - This configuration contains an extra electron in the 2p orbital. According to the Aufbau principle, electrons fill the 2s orbital before filling the 2p orbital. Therefore, this configuration is not a ground state electron configuration.

In conclusion, the correct answer is D. 1s2 2s2 2p1, as it is not a ground state electron configuration.