ACS Chemistry Practice Exam 2025 – The All-in-One Guide to Exam Success!

An electron configuration indicates how electrons are distributed among the various atomic orbitals. In a ground state configuration, electrons fill the lowest available energy levels fully before moving to higher levels, in accordance with the Aufbau principle.

The excited state of an atom occurs when one or more electrons are promoted to higher energy orbitals rather than occupying the lowest energy state available. In the case of the given choices, the configuration "1s2 2s1 2p1" shows that one electron from the 2s orbital has been excited to the 2p orbital, creating an unfilled 2s subshell and a partially filled 2p subshell. This distribution is characteristic of an excited state, as it deviates from the completely filled lower energy orbitals.

In contrast, the other configurations are examples of either ground state configurations or configurations that do not illustrate a promotion of electrons:

- The configuration "1s2 2s2" represents a fully filled 1s and 2s, indicating a ground state.

- The configuration "1s2 2p2" also represents a ground state as both 1s and 2p are filled according to the expected distribution.

- The configuration "1s2 2