Chapter 11: Introduction to Modern Atomic Theory

When we go to helium where do we place the second electron?

The rule is we write the electron configuration for the atom in its lowest energy state. So we must place the second electron in the orbital with the lowest energy. An orbital in the second level (n = 2) is higher in energy compared to placing the second electron into the same orbital that we placed the first electron.

Helium has an electron configuration of 1s². The orbital diagram looks like,

It does take some energy to place two electrons into the same orbital. Recall that electrons are negatively charged. Two electrons in the same orbital repell each other, so it takes some energy to place the second electron into an already half-filled orbital.

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