ACS Chemistry Practice Exam 2026 – The All-in-One Guide to Exam Success!

The ground-state electron configuration of an atom represents the arrangement of electrons in the lowest energy states. Elements in the second period of the periodic table have their electrons filling the 1s and 2s subshells, and eventually the 2p subshell.

An atom in the second period with one valence electron indicates that there is a single electron in the outermost shell. The first shell (1s) can hold two electrons, and once that is filled, the second shell begins to fill. Therefore, for an atom with one valence electron in its ground state, the configuration would be comprised of a filled 1s subshell and a single electron in the 2s subshell.

The configuration 1s2 2s1 correctly illustrates this: the 1s subshell is completely filled with two electrons, while the 2s subshell has one electron. This corresponds to elements like lithium, which is the first in the second period with one valence electron. As the electron configuration accounts for all electrons up to that point, it accurately represents a valence electron situation where only one electron is found in the 2s subshell, leaving the 2p subshell unfilled.

The other configurations