IB Chemistry HLTopic 2 — Models of Bonding & StructurePaper 1 & 2Core skill~11 min read
Molecular Shapes
Molecules are not flat and they are not random. Every shape in this topic comes from one stubborn fact: electron pairs are negative, so they get as far away from each other as they possibly can. Learn to count electron domains and you can predict any shape on the syllabus without memorising a single one.
📘 What you need to know
VSEPR stands for valence shell electron pair repulsion. Electron pairs repel, so they spread out as far apart as possible in 3D.
An electron domain is any region of electron density around the central atom: a single bond, a double bond, a triple bond, or a lone pair.
A multiple bond counts as one domain, not two or three.
Lone pairs repel more strongly than bonding pairs, so they squeeze bond angles closed.
Electron domain geometry counts every domain. Molecular geometry only describes where the atoms are.
Always draw the Lewis formula first — you cannot count lone pairs you have not drawn.
How to count electron domains
This is the only skill on the page. Everything else follows from it.
🧩 The method
Draw the Lewis formula of the species, lone pairs included.
Look only at the central atom. Count every group of electrons attached to it.
One domain for each single bond, each double bond, each triple bond, and each lone pair.
Use the total number of domains to get the electron domain geometry.
Ignore the lone pairs when you name the shape — but remember they are still pushing.
The word “domain” trips people up. Think of it as a lump of negative charge. A triple bond is one fat lump, not three thin ones, so it takes up one position around the central atom.
The six shapes you must know
Two, three or four domains. That is the whole of the standard level list, and it covers the large majority of exam questions.
Two, three and four electron domains
Look down the right-hand column and along the bottom row: same number of domains, different number of lone pairs, and the angle drops every time.
Why lone pairs squash the angles
A bonding pair is stretched between two nuclei, so it is held out at arm’s length. A lone pair belongs to one atom only, so it sits closer in and spreads out more. A fatter, closer lump of charge pushes harder.
Not all repulsions are equal
The pattern is worth learning as a number line: each lone pair takes roughly 2.5° off the bond angle. It is a rule of thumb, but it matches the real values well.
Two different questions, two different answers
Examiners love this distinction because it separates students who understand from students who memorised.
Electron domain geometry answers “where are all the domains?” It counts lone pairs.
Molecular geometry answers “where are the atoms?” It ignores lone pairs, because you cannot see them.
Ammonia has a tetrahedral electron domain geometry but a trigonal pyramidal molecular geometry. Both are correct answers — to different questions.
Bonding pairs
Lone pairs
Total domains
Domain geometry
Molecular geometry
Bond angle
2
0
2
linear
linear
180°
3
0
3
trigonal planar
trigonal planar
120°
2
1
3
trigonal planar
bent
about 118°
4
0
4
tetrahedral
tetrahedral
109.5°
3
1
4
tetrahedral
trigonal pyramidal
107°
2
2
4
tetrahedral
bent
104.5°
🧠 The IB has no single word for the bent shape
Past mark schemes have accepted bent, angular, non-linear and V-shaped. Pick one and stick with it. What you must not write is “trigonal planar” for SO2 or “tetrahedral” for water — those are the domain geometries, not the shapes.
Higher level: expanded octets
Period 3 atoms can hold more than eight electrons, which unlocks five and six domain shapes. The counting method does not change at all.
Bonding pairs
Lone pairs
Domain geometry
Molecular geometry
Example
5
0
trigonal bipyramidal
trigonal bipyramidal
PCl5
4
1
trigonal bipyramidal
seesaw
SF4
3
2
trigonal bipyramidal
T-shaped
ClF3
2
3
trigonal bipyramidal
linear
I3−
6
0
octahedral
octahedral
SF6
5
1
octahedral
square pyramidal
BrF5
4
2
octahedral
square planar
XeF4
Where do the lone pairs go? In a trigonal bipyramid they always take the equatorial positions (the three round the middle), because there is more room there. In an octahedron two lone pairs go opposite each other, which is why XeF4 ends up flat and square.
Worked examples
WE 1
Predict the shape and bond angle of NH3 [3]
Step 1: Lewis formula
N has 5 valence electrons + 3 from H = 8. Three N–H bonds use 6, leaving 1 lone pair.
Step 2: count domains3 bonding + 1 lone = 4 domains → tetrahedral domain geometryStep 3: name the shape and adjust the angle
Ignore the lone pair when naming, but it repels more strongly, so the angle closes from 109.5°.
Trigonal pyramidal, about 107°say “because the lone pair repels more than the bonding pairs” for the third mark
WE 2
Deduce the shape of SO2, which contains a double bond
Step 1: Lewis formula
S has 6 valence electrons, two O attached. One S=O double bond, one S–O bond and 1 lone pair on S.
Step 2: count domains — carefully
The double bond is one domain, not two.
2 bonding domains + 1 lone pair = 3 domainsStep 3: geometry then shape
Three domains → trigonal planar arrangement. One is a lone pair, so the shape is bent.
Bent, about 118°sulfur ends up with 10 electrons here — a legitimate expanded octet
WE 3
State the electron domain geometry and molecular geometry of XeF4
Step 1: count valence electronsXe: 8 + (4 × 7) = 36 electronsStep 2: bonds and lone pairs
Four Xe–F bonds use 8. Each F takes 3 lone pairs = 24. 36 − 8 − 24 = 4 electrons = 2 lone pairs on XeStep 3: total the domains4 bonding + 2 lone = 6 domains → octahedralStep 4: place the lone pairs opposite each other
That leaves the four fluorines in a flat square.
Octahedral domains, square planar moleculetwo answers required — do not give just one
💡 Exam tips
Lewis formula first, every time. Skipping it is how students miss lone pairs and get the shape wrong.
Read whether the question asks for shape or geometry. “Electron domain geometry” and “molecular geometry” are different answers for NH3 and H2O.
Give a reason with the angle. “104.5° because two lone pairs repel more strongly than bonding pairs” is the full answer.
Multiple bonds count as one domain. Check this before you count.
For ions, adjust the electron count for the charge before you count domains.
If you are asked to draw a 3D shape, use wedges and dashes to show which bonds come forward and which go back.
⚠ Common mix-ups
Counting a double bond as two domains. Ethene is trigonal planar, not tetrahedral.
Calling water tetrahedral. Its domains are tetrahedral; the molecule is bent.
Forgetting lone pairs entirely and predicting NH3 is trigonal planar.
Quoting exact angles for lone pair shapes. 107° and 104.5° are the standard values, but write “about” or “approximately”.
Thinking NH4+ is pyramidal because ammonia is. The lone pair has been used up in the coordinate bond, so it is tetrahedral at 109.5°.
Putting lone pairs in axial positions in a trigonal bipyramid. They always go equatorial.
Up next: Bond Polarity — now that you can work out where the atoms sit, you can start asking whether the electrons in each bond are shared fairly.
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