Chapter 11: Introduction to Modern Atomic Theory

The next element is lithium. Lithium has three electrons.

The first two of these three electrons are placed into the 1s orbital.

Where do we place the third electron?

The next lowest energy level is the second level. But there are two sublevels in the second level. Before placing the third electron into the second level the two sublevels are degenerate in energy (have the same energy).

When we place the third electron into the second level the energy of the two sublevels change relative to each other. The 2s sublevel drops in energy and the 2p sublevel increases in energy. So the third electron goes into the orbital in 2s sublevel.

Lithium has an electron configuration of 1s²2s¹. The orbital diagram looks like,

In the orbital diagram we can place each orbital on the same line.

Would like to practice writing an electron configuration?

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