Molecular shapes (e.g. CH₄, NH₃, H₂O, BF₃)

Q: What are the molecular shape and bond angle of methane (\(\text{CH}_4\))?

Methane is tetrahedral with a bond angle of 109.5° .

Q: What are the molecular shape and bond angle of ammonia (\(\text{NH}_3\)), and why is its bond angle smaller than 109.5°?

Ammonia has a trigonal pyramidal shape with a bond angle of approximately 107° because its 1 lone pair exerts stronger repulsion than bonding pairs.

Q: What are the molecular shape and bond angle of water (\(\text{H}_2\text{O}\))?

Water has a bent (or V-shaped) shape with a bond angle of approximately 104.5° .

Q: What are the shapes and bond angles of \(\text{BeCl}_2\) and \(\text{BF}_3\), which have incomplete octets?

\(\text{BeCl}_2\) is linear (180°) and \(\text{BF}_3\) is trigonal planar (120°) .

Q: What are the shapes and bond angles of expanded octet molecules \(\text{PCl}_5\) and \(\text{SF}_6\)?

\(\text{PCl}_5\) is trigonal bipyramidal (90° and 120°) and \(\text{SF}_6\) is octahedral (90°) .

Q: How are multiple bonds (double or triple bonds) treated when determining molecular geometry using VSEPR theory?

A double or triple bond is treated as a single electron domain (region of negative charge) .

Practice

Chemistry · Senior Secondary (HKDSE) · 5 min read · Updated: 21 Aug 2026

Molecular Shapes: Building Molecules in 3D!

Ever wondered why water is written as \(\text{H}_2\text{O}\) and has a bent shape, or why methane is tetrahedral? Molecules aren't flat drawings on a page; they are three-dimensional structures. Understanding their shapes is super important because it helps us predict their polarity, intermolecular forces, boiling points, and chemical reactivity.

In this chapter, we're going to become molecular architects! You'll learn how to predict and draw the 3D shapes of octet molecules, expanded/incomplete octet molecules (such as \(\text{BeCl}_2\), \(\text{BF}_3\), \(\text{PCl}_5\), and \(\text{SF}_6\)), species with multiple bonds, and polyatomic ions.

The Core Idea: Valence Shell Electron Pair Repulsion (VSEPR) Theory

What's with the name?

Let's break down VSEPR:

Valence Shell: We look at the electrons in the outermost shell of the central atom.
Electron Pair / Domain: Valence electrons exist as bonding pairs or lone pairs. Multiple bonds (double/triple bonds) count as a single electron domain when predicting geometry.
Repulsion: Negatively charged electron domains repel each other and spread out as far as possible in 3D space to minimise electrostatic repulsion.

Two Types of Electron Pairs

Bonding Pairs (BPs): Electron pairs shared between the central atom and bonded atoms.
Lone Pairs (LPs): Non-bonding valence electron pairs localized on the central atom.

Important Principle: Lone pairs are held by only one nucleus, so their electron clouds spread out more and exert stronger repulsion than bonding pairs:

Lone Pair - Lone Pair repulsion > Lone Pair - Bonding Pair repulsion > Bonding Pair - Bonding Pair repulsion

5-Step Guide to Predicting Molecular Shapes

Step 1: Identify the central atom.

Step 2: Determine the number of valence electrons on the central atom (equal to group number for main group elements; adjust for charge if dealing with an ion: add electrons for anions, subtract for cations).

Step 3: Determine the number of bonding electron pairs / bonded atoms.

Step 4: Calculate the number of lone pairs (LPs) on the central atom:

\(\text{LPs} = \frac{\text{Valence electrons} - \text{electrons used in bonding}}{2}\)

Step 5: Count the total number of electron domains (bonding domains + lone pairs) to find the electron pair geometry, then deduce the molecular shape from the positions of the atoms.

Case Studies: Octet Molecules

1. Methane (\(\text{CH}_4\))

Central Atom: Carbon (Group 14: 4 valence electrons).
Bonding Pairs: 4; Lone Pairs: 0.
Total Electron Pairs: 4
Molecular Shape: Tetrahedral
Bond Angle: 109.5°
3D Drawing: Use wedge-and-dash notation (solid wedge coming out of the plane, dashed line going into the plane).

2. Ammonia (\(\text{NH}_3\))

Central Atom: Nitrogen (Group 15: 5 valence electrons).
Bonding Pairs: 3; Lone Pairs: 1.
Total Electron Pairs: 4 (tetrahedral electron arrangement).
Molecular Shape: Trigonal Pyramidal
Bond Angle: Approx. 107° (The lone pair repels bonding pairs more strongly, reducing the angle from 109.5°).

3. Water (\(\text{H}_2\text{O}\))

Central Atom: Oxygen (Group 16: 6 valence electrons).
Bonding Pairs: 2; Lone Pairs: 2.
Total Electron Pairs: 4 (tetrahedral electron arrangement).
Molecular Shape: Bent / V-shaped
Bond Angle: Approx. 104.5° (Two lone pairs cause greater repulsion, further compressing the bond angle).

Non-Octet Central Atoms (Without Lone Pairs)

Some central atoms have incomplete octets (fewer than 8 valence electrons) or expanded octets (more than 8 valence electrons using d-orbitals):

4. Beryllium Chloride (\(\text{BeCl}_2\)) — Incomplete Octet

Central Atom: Beryllium (Group 2: 2 valence electrons).
Bonding Pairs: 2; Lone Pairs: 0.
Molecular Shape: Linear
Bond Angle: 180°

5. Boron Trifluoride (\(\text{BF}_3\)) — Incomplete Octet

Central Atom: Boron (Group 13: 3 valence electrons).
Bonding Pairs: 3; Lone Pairs: 0.
Molecular Shape: Trigonal Planar
Bond Angle: 120°

6. Phosphorus Pentachloride (\(\text{PCl}_5\)) — Expanded Octet

Central Atom: Phosphorus (Group 15: 5 valence electrons).
Bonding Pairs: 5; Lone Pairs: 0.
Molecular Shape: Trigonal Bipyramidal
Bond Angles: 90° (axial-equatorial) and 120° (equatorial-equatorial).

7. Sulfur Hexafluoride (\(\text{SF}_6\)) — Expanded Octet

Central Atom: Sulfur (Group 16: 6 valence electrons).
Bonding Pairs: 6; Lone Pairs: 0.
Molecular Shape: Octahedral
Bond Angle: 90°

Molecules with Multiple Bonds & Polyatomic Ions

Molecules with Multiple Bonds

In VSEPR theory, a double or triple bond is treated as a single electron domain/region:

Carbon Dioxide (\(\text{CO}_2\)): Carbon forms two double bonds with oxygen (\(\text{O=C=O}\)). 2 electron domains around C with 0 lone pairs \(\rightarrow\) Linear, bond angle 180°.
Methanal / Formaldehyde (\(\text{HCHO}\)): Carbon forms two single C-H bonds and one C=O double bond. 3 electron domains around C with 0 lone pairs \(\rightarrow\) Trigonal Planar, bond angle approx. 120°.

Polyatomic Ions

Ammonium Ion (\(\text{NH}_4^+\)): Nitrogen has \(5 - 1 = 4\) valence electrons. Forms 4 BPs and 0 LPs \(\rightarrow\) Tetrahedral, bond angle 109.5°.
Hydronium Ion (\(\text{H}_3\text{O}^+\)): Oxygen has \(6 - 1 = 5\) valence electrons. Forms 3 BPs and 1 LP \(\rightarrow\) Trigonal Pyramidal, bond angle approx. 107°.

Summary Table for HKDSE Shapes

Molecule/Ion: \(\text{BeCl}_2\), \(\text{CO}_2\)
Bonding Domains: 2
Lone Pairs: 0
Shape: Linear
Angle: 180°

Molecule/Ion: \(\text{BF}_3\), \(\text{HCHO}\)
Bonding Domains: 3
Lone Pairs: 0
Shape: Trigonal Planar
Angle: 120°

Molecule/Ion: \(\text{CH}_4\), \(\text{NH}_4^+\)
Bonding Domains: 4
Lone Pairs: 0
Shape: Tetrahedral
Angle: 109.5°

Molecule/Ion: \(\text{NH}_3\), \(\text{H}_3\text{O}^+\)
Bonding Domains: 3
Lone Pairs: 1
Shape: Trigonal Pyramidal
Angle: ~107°

Molecule/Ion: \(\text{H}_2\text{O}\)
Bonding Domains: 2
Lone Pairs: 2
Shape: Bent / V-shaped
Angle: ~104.5°

Molecule/Ion: \(\text{PCl}_5\)
Bonding Domains: 5
Lone Pairs: 0
Shape: Trigonal Bipyramidal
Angle: 90°, 120°

Molecule/Ion: \(\text{SF}_6\)
Bonding Domains: 6
Lone Pairs: 0
Shape: Octahedral
Angle: 90°

Key Takeaways

1. Electron Repulsion: Electron domains spread apart to minimise repulsion. Repulsion order: \(\text{LP-LP} > \text{LP-BP} > \text{BP-BP}\).

2. Multiple Bonds: Count double and triple bonds as single electron domains when determining molecular geometry.

3. Electron Pair Geometry vs. Molecular Shape: The arrangement of all electron domains determines the basic geometry, but molecular shape is named only from the positions of bonded atoms.

References

Science Education: Chemistry — Education Bureau, HKSAR Government

Quick check

Can you answer these now?

Open each question to check the key ideas from this chapter.

What is the core principle of Valence Shell Electron Pair Repulsion (VSEPR) theory?

Electron domains (bonding and lone pairs) in the valence shell of a central atom repel each other and spread as far apart as possible in 3D space to minimise electrostatic repulsion.

How does the repulsive strength between lone pairs and bonding pairs compare according to VSEPR theory?

Lone Pair - Lone Pair > Lone Pair - Bonding Pair > Bonding Pair - Bonding Pair.

What are the molecular shape and bond angle of methane (\(\text{CH}_4\))?

Methane is tetrahedral with a bond angle of 109.5°.

What are the molecular shape and bond angle of ammonia (\(\text{NH}_3\)), and why is its bond angle smaller than 109.5°?

Ammonia has a trigonal pyramidal shape with a bond angle of approximately 107° because its 1 lone pair exerts stronger repulsion than bonding pairs.

What are the molecular shape and bond angle of water (\(\text{H}_2\text{O}\))?

Water has a bent (or V-shaped) shape with a bond angle of approximately 104.5°.

What are the shapes and bond angles of \(\text{BeCl}_2\) and \(\text{BF}_3\), which have incomplete octets?

\(\text{BeCl}_2\) is linear (180°) and \(\text{BF}_3\) is trigonal planar (120°).

What are the shapes and bond angles of expanded octet molecules \(\text{PCl}_5\) and \(\text{SF}_6\)?

\(\text{PCl}_5\) is trigonal bipyramidal (90° and 120°) and \(\text{SF}_6\) is octahedral (90°).

How are multiple bonds (double or triple bonds) treated when determining molecular geometry using VSEPR theory?

A double or triple bond is treated as a single electron domain (region of negative charge).

How do you account for charge when determining the shapes of polyatomic ions such as \(\text{NH}_4^+\) and \(\text{H}_3\text{O}^+\)?

Subtract electrons for positive charges (cations) and add electrons for negative charges (anions) when counting central atom valence electrons.

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