3D - 2

Why do electrons go into the 4s orbital before the 3d?

Why do electrons go into the 4s orbital before the 3d? A. The 3d electrons screen the 4s, making it lower in energy. B. The polarizability of the atom makes 3d higher in energy than 4s. C. 3d electrons repel each other more than 4s electrons do. D. Because there are more d-orbitals than s-orbitals, they are higher in energy. E. The s-orbital is more penetrating than the d-orbital, making it closer to the nucleus and lower in energy.

Public Comments

1. E is the correct answer. A is completely wrong, since there aren't any electrons in the 3d to screen the 4s, since electrons go into the 4s first, and besides, screening will not act to lower an outer shell's energy. B and D are both wrong as they have nothing to do with the order electrons fill up in. C may be an effect, but it definitely doesn't play as big a role as E does. E is correct because an electron in the 4s orbital (before any 3d's are added) will spend a good proportion of its time near the nucleus, and hence be low in energy. This is to do with the shape of the 4s orbital (spherical). However, it is important to note that AS SOON AS you add a 3d electron (this first happens with Scandium), then the energies of the 3d and 4s swap over. From then on, the energy of the 3d is lower than that of the 4s. This is becuase of the screening effect the 3d electrons have on the 4s, which results in 4s electrons not spending as much time near the nucleus, and so being higher in energy as a result.