Question #60897

1 Answer
Feb 2, 2017

The energies are almost equal, but slightly different, because the electrons in some orbitals spend more time closer to the nucleus.

Explanation:

In atoms of elements with more than one electron, each principal energy level is split into different sublevels.

For example, the order of increasing energy for orbitals in the fourth level is

4s < 4p < 4d < 4f

upload.wikimedia.orgupload.wikimedia.org

This is because the 4s orbitals have a greater electron density near the nucleus than the 4p orbitals, then come the 4d orbitals, and finally the 4f orbitals.

We can see this if we compare the "radial distribution functions" for each type of orbital, that is, the probability of finding an electron at a given distance from the nucleus.

Radial distributionsRadial distributions
(Adapted from www.f-legrand.fr)

We see that the 4s orbital (bluelline) has a local maximum quite close to the nucleus.

The 4p orbital (purple) has its local maximum further out, and the 4dl(olive) and 4f orbitals (green) have their local maxima still further from the nucleus.

Thus, a 4s electron spends more time close to the nucleus and has a lower energy level.

Next comes the 4p, then the 4d, and the furthest out 4f orbital has the highest energy of the group.