UNIT 2 CHEMISTRY · PEARSON AAQ · B1.5
Molecular Shapes and Bond Angles
FREE CHEMISTRY REVISION · TOPIC 14
Count electron domains, then see the three-dimensional shape.
Predict molecular shapes, explain lone-pair compression and read labelled wedge-and-dash representations with the correct bond angles.
Pearson BTEC Level 3 AAQ Applied Science
Unit 2: Principles and Applications of Chemistry · B1.5
What you will learn
Use electron-pair repulsion to predict shapes with up to six central electron domains; count bonding regions and lone pairs; distinguish electron-domain arrangement from molecular shape; state bond angles; interpret labelled line, wedge and hashed-wedge drawings.
Pearson B1.5 explicitly includes linear, non-linear, trigonal planar, pyramidal, tetrahedral, trigonal bipyramidal and octahedral shapes. Additional lone-pair derivatives below show how the same method applies within the six-domain limit.
1. Electron domains repel
Electron-rich regions around a central atom repel one another and adopt an arrangement that reduces repulsion, usually by spreading as far apart as possible. This is the valence-shell electron-pair repulsion (VSEPR) model. It is a useful prediction model, not a claim that electrons are motionless dots at fixed points.
For geometry, count each single, double or triple bond to an attached atom as one bonding region. Multiple bonds contain more than one shared pair but occupy one direction and repel other regions strongly. Count each central lone pair as one additional region.
A lone pair is concentrated around one nucleus rather than shared between two. It generally occupies more angular space and repels neighbouring regions more strongly. The usual comparison is lone pair–lone pair > lone pair–bonding pair > bonding pair–bonding pair. Lone pairs often compress the angles between bonds.
The electron-domain arrangement includes lone pairs. The molecular shape names the positions of atoms only. Thus NH₃ has four domains in an approximately tetrahedral arrangement but a trigonal pyramidal molecular shape.
2. A reliable counting method
1. Draw a suitable Lewis structure and identify the central atom.
2. Count its bonding regions, grouping each multiple bond into one region.
3. Add the central lone pairs.
4. Choose the electron-domain arrangement for that total.
5. Name the arrangement of the bonded atoms, excluding lone pairs from the shape name.
6. State ideal angles or appropriate compressed values, and explain any compression.
For simple single-bond hydrides and halides, a useful electron-bookkeeping shortcut is to add the central valence electrons and one electron per attached monovalent atom, add an electron for each negative charge and subtract one for each positive charge, then divide by two to count central pairs. Use the Lewis structure when multiple bonds or more complicated bonding are present. Do not apply the shortcut blindly to CO₂.
AXₘEₙ notation is optional: A is the central atom, X an attached atom, and E a lone pair on A. A multiple bond still counts as one X direction in this shape notation.
3. The parent arrangements
| Total regions | Arrangement | Ideal angles | No-central-lone-pair example |
|---|---|---|---|
| 2 | Linear | 180° | CO₂ (two double-bond regions) |
| 3 | Trigonal planar | 120° | BF₃ |
| 4 | Tetrahedral | 109.5° | CH₄ |
| 5 | Trigonal bipyramidal | 90°, 120°, 180° | Gas-phase PCl₅ |
| 6 | Octahedral | 90°, 180° | SF₆ |
Three domains form a flat triangle, but four domains require a tetrahedron in three dimensions. A square-planar arrangement would leave four bonding regions at 90° and is not the maximum-separation arrangement for four equivalent domains.
4. Tetrahedral methane and the wedge key
CH₄ has four bonding regions and no lone pair on carbon (AX₄). They arrange tetrahedrally; all H–C–H angles are approximately 109.5°. A flat cross is not an accurate shape drawing of methane.
A normal line shows a bond approximately in the paper. A solid triangular wedge widens towards an atom closer to the viewer. A hashed wedge represents a bond pointing away. These marks encode depth; they do not mean different bond types or different strengths.
5. Lone pairs: pyramidal ammonia and bent water
NH₃ has three N–H bonding regions and one lone pair on nitrogen: four domains total (AX₃E). The electron domains are approximately tetrahedral, but the atoms form a trigonal pyramidal shape. The lone pair repels bonding regions more strongly than bonding pairs repel one another, compressing H–N–H angles to about 107°.
H₂O has two O–H bonding regions and two lone pairs on oxygen (AX₂E₂). Four domains again give an approximately tetrahedral electron arrangement, but the molecule is non-linear (bent or V-shaped). The H–O–H angle is about 104.5°. With two lone pairs, compression is greater than in NH₃.
The three atoms of water lie in a plane. Showing lone-pair directions with depth symbols does not make the H₂O molecule non-planar. “Non-linear” describes its bent bond arrangement, not an assertion that its atoms leave a plane.
A bent shape can also arise from three electron domains, such as a model with two bonding regions and one central lone pair (AX₂E). Its angle is generally below 120°, not automatically 104.5°. Predict the parent arrangement before assigning a numerical angle.
6. Five domains: trigonal bipyramidal
Five equivalent bonding regions form a trigonal bipyramid. Three equatorial bonds occupy a plane at 120° to one another. Two axial bonds point above and below that plane: 90° to each equatorial bond and 180° to each other.
In gaseous molecular PCl₅ there are five bonding regions and no central lone pair. The axial and equatorial sites are not equivalent. A lone pair in a five-domain arrangement usually occupies an equatorial site, where it has only two 90° interactions; an axial site would give three 90° interactions with equatorial bonds.
This explains derivatives such as SF₄ (AX₄E, seesaw), ClF₃ (AX₃E₂, T-shaped) and XeF₂ (AX₂E₃, linear). The lone pairs occupy equatorial positions in these introductory models; the remaining bonded atoms determine the shape. Lone pairs can distort ideal angles.
7. Six domains: octahedral
Six equivalent bonding regions form an octahedron. Four bonds can be viewed as a square plane around the centre and two as perpendicular axial bonds. Adjacent bonds make 90° and opposite bonds 180°. SF₆ has six S–F bonding regions and no central lone pair (AX₆).
The name refers to the eight triangular faces of the geometrical octahedron, not eight bonds. SF₆ has six bonded atoms around sulfur. All six ideal octahedral positions are equivalent even though a chosen drawing labels an axis.
With one lone pair, six domains give a square-pyramidal shape (AX₅E, for example BrF₅). With two lone pairs, they occupy opposite sites to minimise repulsion; AX₄E₂ (for example XeF₄) is square planar, with 90° and 180° F–Xe–F angles in the ideal model. Square planar here comes from six domains including two lone pairs, not from four equivalent bonding domains alone.
8. Shape and lone-pair checklist
| Example | Bonding regions | Central lone pairs | Molecular shape | Angles |
|---|---|---|---|---|
| CO₂ | 2 | 0 | Linear | 180° |
| BF₃ | 3 | 0 | Trigonal planar | 120° |
| CH₄ | 4 | 0 | Tetrahedral | 109.5° |
| NH₃ | 3 | 1 | Trigonal pyramidal | ≈107° |
| H₂O | 2 | 2 | Bent/non-linear | ≈104.5° |
| PCl₅ (gas) | 5 | 0 | Trigonal bipyramidal | 90°, 120°, 180° |
| SF₆ | 6 | 0 | Octahedral | 90°, 180° |
| XeF₂ | 2 | 3 | Linear | 180° |
| XeF₄ | 4 | 2 | Square planar | 90°, 180° |
NH₃ and H₂O angles are typical approximate values, not exact universal deductions from pair counts. The model predicts the trend; measurements and more detailed bonding descriptions give precise values.
Worked examples
Example 1 · CO₂: multiple bonds count as regions
Lewis structure O=C=O has two double bonds and no lone pair on central C. Each double bond counts as one region for shape. Two regions spread to 180°, so CO₂ is linear. Counting four shared pairs as four spatial directions would wrongly predict a tetrahedron.
Example 2 · BF₃: a trigonal planar centre
B has three bonding regions and no lone pair. Three regions adopt a plane with 120° separation. Name the shape trigonal planar and state the angles. Boron’s six-electron Lewis valence shell does not create a hidden lone pair.
Example 3 · CH₄: explain tetrahedral geometry
Four bonding regions repel equally and spread in three dimensions. The tetrahedral arrangement gives approximately 109.5° between bonds. Draw two appropriate line bonds and use solid and hashed wedges to show the other depth directions, with a key.
Example 4 · NH₃: count and explain compression
N contributes five valence electrons and three H contribute one each: 8 electrons = four central pairs in this simple single-bond model. Three pairs make N–H bonds and one is lone. The atoms are trigonal pyramidal; stronger lone-pair repulsion compresses the angle to about 107°.
Example 5 · H₂O: compare with ammonia
O contributes six valence electrons and two H contribute two in total: 8 = four pairs. Two are bonding and two lone. The molecule is bent. Two lone pairs compress the H–O–H angle to about 104.5°, smaller than NH₃’s approximately 107°.
Example 6 · NH₄⁺: include the ionic charge
5 from N + 4 from H − 1 for the positive charge = 8 valence electrons. All four pairs are bonding; there is no N lone pair. NH₄⁺ is tetrahedral with about 109.5° angles, not pyramidal. A coordinate formation route does not change this count.
Example 7 · PCl₅ and SF₆: name every angle type
PCl₅ gas has five bonding regions: three equatorial and two axial. State 120° equatorial, 90° axial–equatorial and 180° axial–axial. SF₆ has six regions and an octahedral shape: adjacent 90°, opposite 180°.
Example 8 · XeF₄: why four bonds do not always mean tetrahedral
The central Xe has four bonding regions and two lone pairs, giving six domains. Start from octahedral, place the two lone pairs opposite, then name only the bonded-atom positions. The result is square planar, not tetrahedral.
Common misconceptions and exam tips
“Count every shared pair as a separate direction.” A multiple bond is one region for geometry.
“NH₃ is tetrahedral.” Its domain arrangement is approximately tetrahedral; its molecular shape is pyramidal.
“All bent molecules have a 104.5° angle.” The parent domain arrangement and bonding matter.
“The wedge is a stronger bond.” It indicates depth only.
“Octahedral means eight bonds.” There are six positions around the centre.
“Four bonded atoms always mean tetrahedral.” Check central lone pairs; XeF₄ is square planar.
In an explanation, state the bonding-region count, lone-pair count, repulsion argument, molecular shape and bond angle. Label lone pairs clearly. When drawing 3D shapes, include a wedge key and avoid measuring perspective angles from the page.
Your revision summary
Count bonding regions and central lone pairs, then spread the domains to minimise repulsion. Name the molecular shape using only the bonded atoms. Parent arrangements for two to six domains are linear, trigonal planar, tetrahedral, trigonal bipyramidal and octahedral. Lone pairs modify the molecular shape and often compress bonding angles; wedge-and-dash notation communicates depth.
Check your understanding
Choose an answer, select Check Answer and read the scientific explanation. Try Again clears the choice and feedback. Keyboard: Tab between controls, arrow keys within radio groups, Enter or Space on buttons. Answers are not saved.