Why is water bent and not straight?

Oxygen has two lone pairs as well as two bonds. Four regions of electron density repel into a tetrahedral arrangement, and the two lone pairs squeeze the H-O-H angle to about 104.5°.

Why does a lone pair push harder?

It is held by one nucleus only, so it spreads out more and takes up more room close to the central atom than a pair shared between two nuclei.

What is a dot-and-cross diagram for?

It shows which atom each bonding electron came from, so you can check that every atom has ended up with a full outer shell.

3D Molecular Lab - Build & Explore Molecule Shapes

What this experiment shows

Molecules are three-dimensional, and their shape follows from the electron pairs around the central atom repelling one another as far apart as they can get. Methane is tetrahedral, ammonia pyramidal because of its lone pair, water bent for the same reason with two. Being able to turn a molecule and count bonding pairs is the difference between remembering a shape and understanding why it has to be that shape.

CH₄ tetrahedral · NH₃ pyramidal · H₂O bent

What you need

A molecule builder on screen - rotate, zoom and take bonds apart
A periodic table, for the number of outer electrons

How it is done

Pick a molecule and count the outer-shell electrons on the central atom.
Work out how many are bonding pairs and how many are lone pairs.
Arrange all the pairs as far apart as possible.
Name the shape from the positions of the atoms, not the lone pairs.
Turn the model and check it against the dot-and-cross diagram.

What you should see

Four bonding pairs give a tetrahedron, about 109.5°.
Three bonding pairs and one lone pair give a pyramid, about 107°.
Two bonding pairs and two lone pairs give a bent molecule, about 104.5°.
A lone pair pushes harder than a bonding pair, so each one closes the angle a little.

Where marks are lost

Naming the shape from the electron pairs instead of the atoms. Water is bent, not tetrahedral.
Drawing dot-and-cross diagrams with the wrong number of outer electrons.
Forgetting that a double bond counts as one region of electron density.
Saying a molecule is "flat" when it is drawn flat on paper.

Questions students ask

Why is water bent and not straight?: Oxygen has two lone pairs as well as two bonds. Four regions of electron density repel into a tetrahedral arrangement, and the two lone pairs squeeze the H-O-H angle to about 104.5°.
Why does a lone pair push harder?: It is held by one nucleus only, so it spreads out more and takes up more room close to the central atom than a pair shared between two nuclei.
What is a dot-and-cross diagram for?: It shows which atom each bonding electron came from, so you can check that every atom has ended up with a full outer shell.

Try these next

Written for Cambridge IGCSE Chemistry 0620 and CBSE Class 9-12 by Ajay Shekhawat, founder of MrChemCoach. Run the simulator above, then check yourself against the questions.

Water