# Is there an easy way to remember VSEPR shapes?

Nov 1, 2016

I think you will have to do some memorizing, but most of the shapes you can figure out.

#### Explanation:

The Basic Shapes

The basic shapes depend on the steric number ($S N$) of the central atom.

$S N = \text{(number of bonded atoms) + (number of lone pairs)}$

Let's simplify this to

$\textcolor{b l u e}{\overline{\underline{| \textcolor{w h i t e}{\frac{a}{a}} S N = B A + L P \textcolor{w h i t e}{\frac{a}{a}} |}}} \text{ }$

Here are the 5 basic shapes. They are also called the electron geometries. Memorize them.

• $S N = 2$; Linear (in a straight line)
• $S N = 3$; Trigonal planar (three corners flat; i.e. a triangle)
• SN =4; Tetrahedral (four faces [sides]; a pyramid with a triangular base)
• $S N = 5$; Trigonal bipyramidal (two triangular pyramids stuck together at the base)
• $S N = 6$; Octahedral (eight faces; two square pyramids stuck together at the base)

The Secondary Shapes

These occur when you have both atoms and lone pairs on the central atom. These are often called the molecular shapes.

You can usually figure them out by putting the lone pairs as far from the other electrons as possible.

The number of possible shapes is $S N - 1$.

The shapes are summarized in the chart below.

Here's an interesting video that may also help.