Linear Molecular Shape
A focused revision guide to the linear molecular shape, using BeCl2 as the key example. This page explains why two bonding regions and no lone pairs around a central atom give a 180° bond angle.
- 2A.10-a
- 2A.11-a
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-a understand reasons for the shape of, and bond angles in, BeCl2
Electron Domains and the Linear Shape
Choose a step to watch two electron domains repel to 180°, then see the same linear shape in BeCl₂, CO₂ and XeF₂.
© Dr. Mohammed Al-Fatah – onlinelearningsystem.net
GCSE Recap: Shared Pairs and Lone Pairs
Before you start, check the GCSE ideas about shared and lone pairs that this page builds on.
Watch: why electron pairs end up opposite
Two negative charges locked on a ring push each other away. Where do they settle?
Why Electron Pairs Repel
All electrons carry a negative charge. Electron pairs therefore repel one another and arrange themselves as far apart as possible around a central atom.
This is the basis of the electron-pair repulsion model used to explain molecular shapes. The shape depends on how many bonding pairs and lone pairs of electrons surround the central atom.
Key idea: molecular shape is controlled by electron-pair repulsion. Electron pairs move to positions of minimum repulsion.
From Electron Domains to Molecular Shape
A simple two-dimensional circle helps show why two electron domains become opposite each other. Real molecules, however, exist in three-dimensional space.
When electron domains are allowed to arrange themselves in three dimensions, they spread out around an imaginary sphere centred on the central atom. With only two electron domains, the furthest-apart arrangement is still opposite sides of the central atom.
Electron domain: a region of electron density around a central atom. A single bond, double bond, triple bond or lone pair each counts as one electron domain for shape prediction.
Two regions get as far apart as possible at 180°, three regions at 120° in one plane.
Quick Check: Count the Electron Domains
Count the electron domains around the central atom in each molecule.
The Linear Shape
A molecule is linear when there are two bonding regions and no lone pairs around the central atom.
A double bond or a triple bond counts as one bonding region, which is why CO2 and HCN are linear even though the central atom holds four bonding pairs.
The two bonding regions repel equally. The lowest-repulsion arrangement places them in opposite directions, giving a bond angle of 180°.
| Shape | Bonding pairs | Lone pairs | Bond angle | Examples |
|---|---|---|---|---|
| Linear | 2 | 0 | 180° | CO2, CS2, HCN, BeF2, BeCl2 |
Quick Check: Which Species Are Linear?
Click every species that is linear.
Linear Shape in BeCl2
In BeCl2, beryllium is the central atom and chlorine atoms are bonded on either side. The central beryllium atom has two bonding pairs and no lone pairs in the shape model.
Beryllium has only 2 outer-shell electrons, so it finishes with four electrons around it and never reaches an octet.
That does not change the method: the shape follows from the number of electron regions around the central atom. These notes describe gaseous BeCl2, which is the form used in shape questions.
The two Be-Cl bonding pairs repel equally and move as far apart as possible. This gives a linear molecule with a Cl-Be-Cl bond angle of 180°.
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 BeCl2: beryllium has 2 outer-shell electrons, the molecule has no charge, and the two Be-Cl single bonds use both electrons. None are left over, so there are no lone pairs. Two bonding regions and no lone pairs give a linear molecule.
Exam sentence: BeCl2 has two bonding pairs and no lone pairs around Be. The bonding pairs repel equally and move as far apart as possible, so the molecule is linear with a 180° bond angle.
Beryllium has 2 outer electrons, both used in bonds: two regions and no lone pairs give a linear molecule.
Quick Check: Who Has Predicted NO₂⁺ Correctly?
Choose the student with the correct shape and the correct reasoning.
BeCl2 is linear because the two bonding pairs around the central Be atom repel equally and sit opposite one another.
How to Explain Shape in an Exam
Shape explanations need a clear sequence. State the number of electron pairs first, then explain repulsion, then give the shape and bond angle.
1. Count electron pairs
State the number of bonding pairs and lone pairs around the central atom.
2. Use repulsion language
State that electron pairs repel and arrange themselves as far apart as possible.
3. Compare pair types where needed
If there are no lone pairs, state that the bonding pairs repel equally. If there are lone pairs, state that lone pairs repel more strongly than bonding pairs.
4. Give the final shape and angle
For BeCl2, the final answer is linear, 180°.
BeCl2 Exam Answer Model
Use the exam-answer structure from Part 5, then change only the molecule-specific information. The black wording is the reusable sentence frame. The coloured wording is the part that changes for each molecule.
Model answer for BeCl2
There are two bonding pairs and no lone pairs around the central beryllium atom.
Electron pairs repel each other and arrange themselves as far apart as possible to minimise repulsion.
Because there are no lone pairs, the two bonding pairs repel equally.
The molecule is therefore linear with a bond angle of 180°.
Reusable sentence frame
There are [number of bonding pairs] and [number of lone pairs] around the central [central atom].
Electron pairs repel each other and arrange themselves as far apart as possible to minimise repulsion.
If there are [no lone pairs], the bonding pairs repel equally. If there are lone pairs, the lone pairs repel more strongly than bonding pairs.
The molecule is therefore [shape] with a bond angle of [bond angle].
Colour key
Blue = bonding-pair information. Red = lone-pair information. Gold = central atom. Green = final shape and bond angle.
Quick Check: Explain a New Pair of Molecules
Write a short explanation, then compare it with the mark points and the model answer.
Quick Check: The Whole Page in Five Statements
In each round, pick the one accurate statement to review everything on this page.
Common Exam Points
Linear shape questions are usually straightforward, but marks are often lost when students describe the atoms moving apart instead of the electron pairs repelling.
Focus on electron pairs
The cause of the shape is repulsion between regions of electron density, not a simple repulsion between atoms.
State no lone pairs clearly
For BeCl2, the central Be atom has two bonding pairs and no lone pairs in the molecular shape model.
Use the exact angle
The bond angle in a linear molecule is 180°. Do not write approximately 180° unless the question specifically allows approximation.
Do not confuse linear with bent
A bent molecule has lone pairs on the central atom: H2O has two bonding pairs and two lone pairs, so its bond angle is about 104.5°.
A linear molecule has two bonding regions and no lone pairs, counting a double or triple bond as one region, so the angle is 180°.
QuickSnap
The linear shape is produced when a central atom has two bonding regions and no lone pairs. A double or triple bond counts as one region. The two regions repel equally and move to opposite sides of the central atom.
Electron-pair repulsion
Electron pairs repel because electrons are negatively charged.
Linear condition
Two bonding regions and no lone pairs around the central atom. A double or triple bond is one region.
Bond angle
The two bonding regions are furthest apart at 180°.
Example
BeCl2 is linear, with a Cl-Be-Cl bond angle of 180°.
FAQs
These questions target the common points students confuse when learning the linear molecular shape.
What makes a molecule linear?
A molecule is linear when the central atom has two bonding regions and no lone pairs. The bonding pairs repel equally and arrange themselves 180° apart.
Why is BeCl2 linear?
BeCl2 has two Be-Cl bonding pairs around the central Be atom and no lone pairs in the shape model. The two bonding pairs repel equally, so the Cl-Be-Cl bond angle is 180°.
Do lone pairs affect the linear shape?
Yes. Lone pairs repel more strongly than bonding pairs and can reduce bond angles. BeCl2 is linear because there are no lone pairs on the central atom in the shape model.
What bond angle should I write for a linear molecule?
The bond angle for a linear molecule is 180°.
Can a molecule with lone pairs on the central atom be linear?
Yes. XeF2 has two bonding pairs and three lone pairs. The five regions take a trigonal bipyramidal arrangement with all three lone pairs equatorial, so the two fluorine atoms sit axially at 180°.
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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