HS Chemistry - Essentials

Electron Repulsion & Shielding

Overview of The Page

This page will cover:

• What is electron repulsion? How does it affect electron arrangement?
• What is electron shielding? How does it affect electronegativity?

Each electron has a negative charge, and so electrons repel, rather than attract, one another.

A past page covered how box notation shows how the electrons are distributed in the orbitals. For example, Argon's $Ar$ box notation would be:

Following those same rules, Aluminum's box notation would be:

If we were to draw Silicon's box notation, it would look like:

This raises a question. Why did the electron go into another orbital in the 3p subshell, rather than fil the first orbital?

It's due to the fact that electrons repel each other due to being similarly charged particles. The atom will try to be in the most stable state possible, and since electrons in the same orbital will repel each other while electrons in different orbitals won't, the electrons will move into different orbitals in the same subshell if there are any empty orbitals left.

Only after every orbital holds at least one electron do additional electrons start filling up the orbitals. For example, Sulfur's box notation is:

One other thing that should be noted is that if only one more electron is needed to ensure that every orbital present is occupied/filled, then one electron will leave the outermost s subshell and instead go to the lone empty orbital. This is because the atom will be more stable if every orbital has at least one electron, and if only one electron is moved, the s subshell is not left without electrons.

Electrons not only repel other electrons in the same orbital, but they also repel each electrons in other shells. Electrons in inner shells repel electrons in outer shells, canceling some of the nucleus's attractive force, or shielding the outer electrons from it. This phenomenon is known as electron shielding, and the amount of attractive force canceled/shielded from one shell to the next is equal to the amount of electrons in the lower shell.

For example, Calcium, element 20, has 20 protons and 20 electrons It has 2 electrons in the first shell, 8 electrons in the second shell, 8 electrons in the third shell, and 2 electrons in the fourth shell. On the first shell, the nucleus exerts an attractive force of +20. On the second shell, the nucleus exerts an attractive force of +$20 \- 2$, or +18, due to electron shielding. On the third shell, the nucleus exerts an attractive force of +$18 \- 8$, or +10, again due to electron shielding. Finally, on the fourth shell, the nucleus exerts an attractive force of +$10 \- 8$, or +2.

Notice how due to electron shielding, Calcium's 2 valence electrons experience an attractive force of +2. This helps explain the trend noted on the previous page of non-metals having higher electronegativity than metals.