CHEMISTRY COMMUNITY

how do you determine the amount of regions of electron density?

thank you

The number of regions of electron density correspond to the number of lone pairs or bonding pairs an atom has. For example, in CH4 (methane), there are 4 regions of electron density because there are 4 bonding pairs (or 4 bonds C makes with the H's). In H2O, oxygen has 4 regions of electron density as well because while O makes 2 bonds with the 2 H's, the atom also has two lone pairs. I hope this made sense!

Here's a breakdown of what is considered an area of electron density.

1 bond = 1 region of electron density
1 double bond = 1 region of electron density
1 lone pair = 1 region of electron density

After determining the structure of the molecule, count these up to find the number of areas of electron density.

You determine the number of regions of electron densities by the electrons and bonds. If there is a lone pair, this counts as one region of electron density. If there is one electron (like a radical), this also counts as one region of electron density. All bonds (single, double, triple) count as one region of electron density.

So, for NH3, there are three N-H bonds and one lone pair. Therefore, there are 4 regions of electron density.

But remember, the name of the shape of the molecule is based on the number of atoms.

To figure out regions of electron density you essentially just have to look at the number of lone pairs and atoms around the atom you are looking at. Single, double, and triple bonds are all considered one region of electron density and each lone pair is considered one region of electron density.

Just to add on, an unpaired electron on a radical like NO2 is also considered to be a region of electron density.

The number of regions of electron density has to do with how many lone pairs and bonding areas there are (single, double, and triple bonds are counted as one bonding region). CH₄ has 4 bonds, therefore, 4 regions of electron density. Likewise, NH₃ has 3 bonds and 1 lone pair on nitrogen, making up 4 regions of electron density as well. Hope this helps!

Adding onto the comments above me, you would count bonds and electron pairs as one region of electron density each. For example, it does not matter whether a bond is a single, double, or triple bond; all of them count as a single region of electron density. Likewise, a single unpaired electron (like in a radical) and a lone pair both count as a single region of electron density :)