Bent Molecular Shape
A focused revision guide to the bent molecular shape, using H2O as the key example. This page explains why two bonding pairs and two lone pairs around a central atom give a non-linear arrangement with a bond angle of about 104.5°.
- 2A.10-a
- 2A.11-f
- 2A.12-a
- 2A.12-b
What these spec points say
- 2A.10-a understand that the shape of a simple molecule is determined by repulsion between electron pairs around a central atom
- 2A.11-f understand reasons for the shape of, and bond angles in, H2O
- 2A.12-a be able to predict the shapes of simple molecules analogous to specified examples using electron-pair repulsion theory
- 2A.12-b be able to predict the bond angles in simple molecules analogous to specified examples using electron-pair repulsion theory
Bent Shape of H₂O
Choose a step to watch four electron pairs form a tetrahedron, two stay as lone pairs and close the bond angle to 104.5°.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
GCSE Recap: Shared Pairs and Lone Pairs
Before you start, check that you can count shared pairs and lone pairs in a dot-and-cross diagram.
What Bent Means
A bent molecule has two atoms bonded to a central atom, with two lone pairs also present on the central atom. The bonded atoms are not arranged in a straight line because the lone pairs repel the bonding pairs and compress the bond angle.
The key example for this page is H2O. Oxygen is the central atom, each hydrogen atom is bonded to oxygen, and oxygen also has two lone pairs.
Key idea: two bonding pairs and two lone pairs around a central atom give a bent shape with a bond angle of about 104.5°.
Why H2O Is Bent
In H2O, the central oxygen atom is surrounded by two O-H bonding pairs and two lone pairs. These four regions of negative charge repel one another and are based on a tetrahedral electron-pair arrangement.
How to count:
- Start with the outer-shell electrons of the central atom.
- Add one electron for each negative charge and take one away for each positive charge.
- Each single bond uses one of those electrons and each double bond uses two. A double or triple bond still counts as one bonding region.
- Divide the electrons left over by two to get the lone pairs.
In H2O: oxygen has 6 outer-shell electrons, the molecule has no charge, and the two O-H single bonds use two of them. Four are left over, which is two lone pairs.
Two bonding regions and two lone pairs give four regions, so the arrangement is tetrahedral and the shape is bent. The same count works for an ion once the charge has been added or taken away.
The molecular shape only considers the positions of the atoms, not the invisible lone pairs. Therefore, H2O is described as bent, not tetrahedral.
A bent species has two bonding regions and two lone pairs around the central atom. H2O is the key model example, with a bond angle of about 104.5°.
Oxygen keeps 4 of its 6 outer electrons as two lone pairs: two bonding regions and two lone pairs give a bent molecule.
Quick Check: Count the Pairs in Hydrogen Selenide
Apply the same reasoning to a molecule that is not on this page, answering each question as quickly as you can.
Watch: two lone pairs squeeze harder
Two bonding pairs and two lone pairs on one atom. What happens to the bond angle?
Bonding Pairs, Lone Pairs and Bond Angle
The four electron regions around oxygen in H2O are arranged to minimise repulsion. If all four regions were bonding pairs, the bond angle would be close to the tetrahedral angle of 109.5°.
However, H2O has two lone pairs. Lone pair-lone pair repulsion is stronger than lone pair-bonding pair repulsion, which is stronger than bonding pair-bonding pair repulsion. This stronger repulsion compresses the H-O-H bond angle to about 104.5°.
Bonding pair-bonding pair
Weakest repulsion of the three common repulsion types.
Lone pair-bonding pair
Stronger repulsion, so bond angles are compressed.
Lone pair-lone pair
Strongest repulsion. NH3 has one lone pair and a bond angle of 107°, while H2O has two and closes to about 104.5°. As a rule of thumb, each lone pair takes about 2.5° off the tetrahedral angle of 109.5°.
Swapping a bonding pair for a lone pair closes the angle by about 2.5° each time: 109.5°, 107°, 104.5°.
Quick Check: Rank the Bond Angles
Drag the four fluorides into order of bond angle, largest at the top.
Bent Shape Examples
The bent shape is found when the central atom has two bonding pairs and two lone pairs. H2O is the standard example used to explain the 104.5° bond angle.
The last column gives the angle predicted at A Level, which is what mark schemes expect. Measured angles vary around it, and H2S is actually about 92°. Quote about 104.5° unless a question gives you a value to use.
| Species | Central atom | Electron regions | Shape | Angle predicted at A Level |
|---|---|---|---|---|
| H2O | O | 2 bonding regions, 2 lone pairs | Bent | about 104.5° |
| OCl2 | O | 2 bonding regions, 2 lone pairs | Bent | about 104.5° |
| H2S | S | 2 bonding regions, 2 lone pairs | Bent | about 104.5° |
| OF2 | O | 2 bonding regions, 2 lone pairs | Bent | about 104.5° |
| SCl2 | S | 2 bonding regions, 2 lone pairs | Bent | about 104.5° |
Quick Check: Find the Bent Species
Count the electron pairs around each central atom and click every species with two bonding pairs and two lone pairs.
How to Explain Bent Shape in an Exam
A full exam explanation should connect the number of electron pairs to electron-pair repulsion, then explain why the bond angle is smaller than the tetrahedral angle.
1. Identify the central atom
For H2O, the central atom is oxygen.
2. Count bonding pairs and lone pairs
Oxygen has two bonding pairs and two lone pairs around it.
3. Apply electron-pair repulsion
The electron pairs repel and arrange themselves as far apart as possible, but the two lone pairs repel more strongly than bonding pairs.
4. State the shape and angle
The molecule is bent with a bond angle of about 104.5°.
Exam answer model: H2O has two bonding pairs and two lone pairs around the central oxygen atom. The electron pairs repel and arrange themselves as far apart as possible.
The two lone pairs repel the bonding pairs more strongly, compressing the H-O-H bond angle from 109.5° to about 104.5°. Therefore, H2O is bent.
Quick Check: Explain the Amide Ion
Write a short exam-style explanation, then compare it with the mark points and the model answer.
Quick Check: Whose Reasoning Is Sound?
In each round, choose the one student whose reasoning is correct.
Common Exam Points
Do not call H2O linear
Although there are only two bonded atoms, the two lone pairs on oxygen bend the molecule.
Separate electron-pair arrangement from molecular shape
The electron-pair arrangement is based on tetrahedral geometry, but the molecular shape is bent because only atoms are used to describe the visible shape.
Use the correct repulsion order
Lone pair-lone pair repulsion is strongest, followed by lone pair-bonding pair, then bonding pair-bonding pair.
Count around the central atom only
The shape is determined by the bonding pairs and lone pairs around the central atom, not by the total number of atoms in the whole species.
QuickSnap
The bent shape is produced when a central atom has two bonding pairs and two lone pairs. The lone pairs repel bonding pairs more strongly and compress the bond angle to about 104.5°.
Memory line: 2 bonding pairs + 2 lone pairs = bent = about 104.5°.
FAQs
These are the key questions students often ask when revising the bent molecular shape.
What makes a molecule bent?
A molecule is bent when the central atom has two bonded atoms and lone pairs that repel the bonding pairs. In the standard H2O example, oxygen has two bonding pairs and two lone pairs.
Why is H2O bent?
H2O is bent because oxygen has two O-H bonding pairs and two lone pairs. The lone pairs occupy electron regions but are not shown as atoms in the molecular shape.
What is the bond angle in H2O?
The H-O-H bond angle in H2O is about 104.5°. It is smaller than 109.5° because the two lone pairs repel bonding pairs more strongly.
Is bent the same as linear?
No. A linear molecule has a bond angle of 180°. A bent molecule has a smaller bond angle because lone pairs on the central atom repel the bonding pairs.
Why is the bond angle in H2O smaller than in NH3?
H2O has two lone pairs and NH3 has one. Lone pair-lone pair repulsion is the strongest, so the bonding pairs in water are pushed closer together: about 104.5° against 107°.
Copyright notice: This OLS revision content, including the explanations, layout, diagrams, tables and embedded learning structure, is authored for Online Learning System by Dr. Mohammed Al-Fatah. It may not be copied, reproduced, redistributed or adapted without written permission.
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