Determine the element of lowest atomic number that contains a half-filled d subshell in the ground state?

The lowest electron shell with the d orbital is 3

It contains 3s, 3p and 3d

This means the configuration of the electrons will be

1s², 2s², 2p⁶, 3s², 3p⁶, 3d⁵, 4s²

This is because the 4s orbital is at a lower energy state when it has no electrons in it and moves to a higher energy state when it is completely filled. After its transition, the 3d orbital starts filling.

Considering this is an atom in its ground state and not an ion, the number of electrons is equal to the number of protons. Therefore, the lowest atomic number possible is:

25

This element is Manganese