Predicting the shape of any molecule or ion

5 minAS Level3.5.2

AI-drafted, independently checked · not yet teacher-reviewed

By the end of this lesson

By the end you can predict the shape and bond angle of a molecule or ion you've never seen, such as $\text{NH}_4^{+}$ , $\text{H}_3\text{O}^{+}$ , $\text{NH}_2^{-}$ or $\text{PCl}_6^{-}$ .

Seven shapes, endless molecules

You already know seven shapes. Every new molecule or ion in this topic is one of them in disguise.

The trick is to count the bonding pairs and lone pairs on the central atom, then match the pattern.

The lookup table

bonding pairs	lone pairs	shape	VSEPR-predicted bond angle	just like
2	0	linear	180°	$\text{CO}_2$
3	0	trigonal planar	120°	$\text{BF}_3$
4	0	tetrahedral	109.5°	$\text{CH}_4$
3	1	pyramidal	107°	$\text{NH}_3$
2	2	non-linear	104.5°	$\text{H}_2\text{O}$
5	0	trigonal bipyramidal	120° and 90°	$\text{PF}_5$
6	0	octahedral	90°	$\text{SF}_6$

Use these as idealized VSEPR predictions by analogy, not as exact measured angles for every molecule.

The counting method

For a central atom with single bonds to H or halogen atoms:

Start with the central atom's outer electrons (B and Al 3, C and Si 4, N and P 5, O and S 6).
Add 1 for each negative charge; take away 1 for each positive charge.
Take away 1 for each bond (the central atom puts one electron into each).
Lone pairs = electrons left ÷ 2.
Look up bonding pairs + lone pairs in the table.

Quick check

Multiple choiceQuestion 1 · Recall

CalculateQuestion 2 · Apply

Answer the 2 questions above to continue.