homework problem 2A1 part c

[Zaid Bustami 1B]

Hey guys,
So for part c, the question is asking how many valence electrons manganese has. How is it that Mn has 7 valence electrons and not 2, since the 3d electrons are in a lower energy shell than the 4s electrons? If valence electrons are defined as electrons in the highest energy shell, why are the 3d electrons also included?
Thanks :)

[Minh-Thy Vo 3D]

Manganese is a transition metal so it can have more than one valence state. The configuration is [Ar] 3d5 4s2. So the 2 electrons in the 4s orbitals are definitely valence electrons. However, because it is a transition metal, the 5 electrons in the 3d orbital can also be valence electrons. This is because the energy difference between the 4s and 3d orbitals are very small. So overall, Mn can have up to 7 valence electrons. Hope this helps!