The ground-state electron configuration of a neutral atom of manganese is [Ar]3d^54s^2. This means that the manganese atom has 25 electrons, with the first 18 electrons filling the inner shells (1s, 2s, 2p, 3s, and 3p), and the remaining 7 electrons filling the 3d and 4s subshells. The 3d subshell is partially filled, with five electrons, while the 4s subshell is fully filled, with two electrons.