0.6 Electron orbitals and electron configurations in atoms  (Page 8/10)

We can use this principle and the ionization energies to determine the electron configurations of lithium, beryllium, and boron. If beryllium’s configuration was 1s ² 2p ² , we could put a third electron in a 2p orbital, because there are three 2p orbitals, as we recall from above. This would mean that boron’s electron configuration would be 1s ² 2p ³ , and there would be only two ionization energies. But this is not right: the data show that there are three ionization energies for boron. If beryllium’s electron configuration were 1s ² 2s ² , then the added electron in boron would have to go into a new orbital, and boron’s electron configuration would be 1s ² 2s ² 2p ¹ . The data in [link] for boron match this configuration. Notice that it appears that the 2s electrons and the 2p electron have very similar ionization energies. This makes sense, since both have the same n value. We will later explore the reason why they don’t have exactly the same energy.

Does this concept account for the ionization energies of the next several elements (carbon to neon)? In each of these elements, there are only three ionization energies, and just as in boron, one is very large and the other two are smaller and comparable in size. But there are six elements from boron to neon. How can it be that we can have six electrons in the 2p orbital? The answer is that there are three 2p orbitals, so two electrons can move in each orbital for a total of six 2p electrons. For example, the electron configuration of neon would be 1s ² 2s ² 2p ⁶ .

The next obvious step in the data in [link] comes with sodium, where a fourth ionization energy is observed. This means that there are electrons in four different types of orbitals. If our reasoning above is correct, this makes sense. There is no room for a seventh electron to move in the 2p orbitals, so one electron in sodium must be in a higher energy orbital. The next higher energy orbital would be either 3s, 3p, or 3d, since n = 3 is the next lowest energy level. The ionization energies bear this out, since one of them is quite large (the 1s electrons) two of them are moderately sized (the 2s and 2p electrons), and one is much smaller (the n = 3 electron). Just by looking at the pattern of the ionization energies for the elements from sodium to argon, it should be clear that we have the same pattern as for lithium to neon. This means that the same argument must apply, and the 3s orbital must have an electron in sodium and must have two electrons in magnesium. Sodium’s electron configuration must be 1s ² 2s ² 2p ⁶ 3s ¹ and magnesium’s must be 1s ² 2s ² 2p ⁶ 3s ² .

Our ionization energy data have provided us with three conclusions. First we conclude that two, and only two, electrons can move in the same orbital. Second, the electron configuration for each atom can be found by assigning electrons two at a time to each orbital in increasing order of energy, with the s orbital lower in the energy than the p orbital for each n value. This is sometimes called the aufbau principle, after the German word meaning, roughly, “built up.”