Chapter 11: Introduction to Modern Atomic Theory

The next element is beryllium which has four electrons. Where do we place the fourth electron? The 2s and 2p sublevels are very close. Which sublevel do we place the fourth electron? We place it into the 2s sublevel.

Beryllium has an electron configuration of 1s²2s². The orbital diagram looks like,

Does beryllium, with an electron configuration of 1s²2s², have a filled shell? No.

The reason is that all of the orbitals in the second level are not yet full. The 2p sublevel is completely empty.

After beryllium comes boron with five electrons.

The electron configuration for boron is 1s²2s²2p¹. The orbital diagram for boron is...well let's look at the orbital diagram before we place the fifth electron.

The three orbitals in the 2p sublevel are all the same energy, they are degenerate in energy, so it really makes no difference whether we write and orbital diagram as,

or or

Notice in the electron configuration for boron (1s²2s²2p¹) we do not worry about which orbital the electron is located, it is only in the orbital diagram that we must be concerned about such details.

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