0.5 Electron orbitals and electron configurations in atoms  (Page 7/10)

The threshold ionization energies for the first twenty elements are given in [link] .

There is a lot of data in this table, so we should take it line by line and look for patterns. We note that there is a single threshold for hydrogen and helium. In hydrogen, this makes sense, because there is only a single electron. The lowest energy orbital is the 1s orbital, since n = 1 is the lowest value and only l = 0 is possible when n = 1. But helium has two electrons. Why is there only a single type of electron in helium? It must be the case that both electrons in helium are in the same orbital with the same energy. Also, the ionization energy for helium is about double the ionization energy of hydrogen. This makes sense when we remember that the charge on the nucleus is in Coulomb’s law: doubling the charge should double the strength of the attraction of the electron to the nucleus. This very strongly indicates that the two electrons in helium are in the same orbital as the one electron in hydrogen. If we want to describe the helium atom, we could state that it has two 1s electrons. We adopt a shorthand notation for this, 1s ² , meaning that there are two electrons in the 1s orbital. This is called the electron configuration of helium. This is an extremely important conclusion because it tells us that we can use the electron orbitals for the hydrogen atom to describe the motions of electrons in other atoms.

We now look at lithium and beryllium and notice that there are two ionization energies for each, meaning that there are two types of electrons in each atom. In lithium, this definitely means that the electron configuration is not 1s ³ . Apparently, there cannot be three electrons in a 1s orbital, so the third electron must go into a higher energy orbital. The next orbital higher in energy would be either a 2s or a 2p orbital. So lithium must have two electrons in the 1s orbital and one electron in either the 2s or 2p orbital, and the electron configuration is either 1s ² 2s ¹ or 1s ² 2p ¹ . Whichever it is, it appears that beryllium will have a similar electron configuration 1s ² 2s ² or 1s ² 2p ² , since there are only two ionization energies for beryllium. And we already know that two electrons can be in the same orbital. But which is the correct electron configuration?

To find out, we look at boron and notice that suddenly there are three ionization energies. One of them is much larger than the other two, and the other two are fairly similar. Certainly the large ionization energy is due to two electrons in the 1s orbital. It seems probable that one of the other two ionization energies is for an electron in the same orbital as lithium or beryllium. The third one represents an electron in a new orbital. Apparently, we cannot put a third electron into whichever orbital beryllium has two electrons in. This is just what we saw before: there appears to be a fundamental principle that only two electrons can move in the same electron orbital. This principle is part of a more general principle called the “Pauli Exclusion Principle,” named after its discoverer.