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Methane (CH₄), ammonia (NH₃) and water (H₂O) are three of the most important small molecules on Earth. They sit next to each other in the second period — carbon, nitrogen, oxygen — and they have something in common: each central atom is surrounded by four pairs of electrons. Yet they have three different shapes, three different bond angles and very different properties. Comparing them side by side is one of the clearest ways to see how lone pairs control molecular structure and behaviour.
The comparison at a glance
| Feature | Methane, CH₄ | Ammonia, NH₃ | Water, H₂O |
|---|---|---|---|
| Central atom | Carbon (group 14) | Nitrogen (group 15) | Oxygen (group 16) |
| Bonding pairs | 4 | 3 | 2 |
| Lone pairs | 0 | 1 | 2 |
| Total electron pairs | 4 | 4 | 4 |
| Electron geometry | Tetrahedral | Tetrahedral | Tetrahedral |
| Molecular shape | Tetrahedral | Trigonal pyramidal | Bent (V-shaped) |
| Bond angle | 109.5° | about 107° | about 104.5° |
| Hybridisation | sp³ | sp³ | sp³ |
| X–H bond length | 109 pm | 101 pm | 96 pm |
| Electronegativity of centre | 2.55 | 3.04 | 3.44 |
| Polar molecule? | No | Yes (1.47 D) | Yes (1.85 D) |
| Hydrogen bonding between molecules? | No | Yes | Yes (strongest) |
| Boiling point | −162 °C | −33 °C | 100 °C |
| Behaviour in water | Insoluble | Very soluble; weak base | — |
Same electron arrangement
In all three molecules, the central atom has eight outer electrons in four pairs (see covalent bonding explained):
- Carbon has 4 valence electrons and forms 4 bonds → 4 bonding pairs, 0 lone pairs.
- Nitrogen has 5 valence electrons and forms 3 bonds → 3 bonding pairs, 1 lone pair.
- Oxygen has 6 valence electrons and forms 2 bonds → 2 bonding pairs, 2 lone pairs.
According to VSEPR theory, four pairs of electrons arrange themselves tetrahedrally to minimise repulsion (see VSEPR and molecular geometry). So all three have a tetrahedral electron geometry — and all three central atoms are described as sp³ hybridised (see hybridisation explained).
Different shapes
The shapes differ because we name shapes by where the atoms are, not the lone pairs (see electron geometry vs molecular geometry):
- Methane: all four corners of the tetrahedron are hydrogen atoms → tetrahedral.
- Ammonia: three corners are hydrogen atoms and one is a lone pair → the N and three H form a pyramid with a triangular base.
- Water: two corners are hydrogen atoms and two are lone pairs → the molecule is bent.
Different bond angles
Lone pairs are held by only one nucleus, so they sit closer to the central atom and spread out more than bonding pairs. They repel more strongly and squeeze the bonds together:
- CH₄: no lone pairs → the ideal tetrahedral angle, 109.5°.
- NH₃: one lone pair → angle reduced to about 107°.
- H₂O: two lone pairs (and lone pair–lone pair repulsion is strongest) → reduced further to about 104.5°.
Each lone pair reduces the angle by roughly 2–2.5° (see bond angles explained and how lone pairs change molecular shape).
Polarity
Methane: non-polar
The C–H bond is only slightly polar (Δχ = 2.55 − 2.20 = 0.35). Even so, in a perfect tetrahedron the four bond dipoles cancel exactly. Methane has no dipole moment.
Ammonia: polar
N–H bonds are clearly polar (Δχ = 0.84), with nitrogen δ−. In the pyramid, the three bond dipoles all point partly towards nitrogen, and the lone pair adds to the effect. Ammonia has a dipole moment of 1.47 D.
Water: most polar
O–H bonds are more polar still (Δχ = 1.24). In the bent shape, the bond dipoles add up, reinforced by two lone pairs. Water’s dipole moment is 1.85 D (see dipole moments and bond polarity).
Hydrogen bonding and boiling points
Hydrogen bonding requires a hydrogen attached to N, O or F and a lone pair on a nearby N, O or F atom (see hydrogen bonding).
- Methane has neither polar X–H bonds nor lone pairs. Its molecules attract each other only by weak London forces, so it boils at −162 °C.
- Ammonia forms hydrogen bonds, but each molecule has three δ+ hydrogens and only one lone pair. On average, each molecule can form only about two hydrogen bonds (limited by the lone pairs). Boiling point: −33 °C.
- Water has two δ+ hydrogens and two lone pairs — a perfect match. Each molecule can form up to four hydrogen bonds, building an extensive network. Boiling point: 100 °C.
Their molar masses are almost identical (16, 17 and 18 g mol⁻¹), so the huge differences in boiling point come almost entirely from shape, polarity and hydrogen bonding (see intermolecular forces).
Chemical behaviour
The lone pairs don’t just shape the molecules; they also control how they react.
- Methane has no lone pairs and strong, non-polar C–H bonds. It’s unreactive at room temperature, except that it burns in oxygen, releasing a lot of energy — it’s the main component of natural gas.
- Ammonia’s lone pair can accept a proton, making it a base: NH₃ + H₂O ⇌ NH₄⁺ + OH⁻. When it gains H⁺, it becomes the ammonium ion, NH₄⁺ — which has four bonding pairs and no lone pairs, so it’s a perfect tetrahedron with 109.5° angles (see dative covalent bonds).
- Water’s lone pairs let it act as a base too (forming H₃O⁺, which is trigonal pyramidal like ammonia), while its polar O–H bonds let it act as an acid (forming OH⁻, see amphoteric substances).
The isoelectronic pattern
CH₄, NH₃, H₂O, HF and the neon atom all have 10 electrons in total — they’re isoelectronic. Moving along the series, protons are moved from hydrogen nuclei into the central nucleus. This pattern makes a neat sequence:
| Species | Bonding pairs | Lone pairs | Shape |
|---|---|---|---|
| CH₄ | 4 | 0 | Tetrahedral |
| NH₃ | 3 | 1 | Trigonal pyramidal |
| H₂O | 2 | 2 | Bent |
| HF | 1 | 3 | Linear (diatomic) |
| Ne | 0 | 4 | Single atom |
The related ions follow too: NH₄⁺ (tetrahedral), H₃O⁺ (pyramidal), OH⁻ (linear diatomic).
Why this comparison matters
- It shows that lone pairs decide shape, even though you can’t “see” them in the final structure.
- It connects shape → polarity → intermolecular forces → physical properties in one clear chain.
- It explains everyday facts: why natural gas is a gas, why ammonia is a pungent gas that dissolves in water to make a cleaning solution, and why water is a liquid essential for life.
Common misconceptions
- “Ammonia is trigonal planar.” Its lone pair makes it a pyramid.
- “All three are tetrahedral.” Only their electron geometries are; their molecular shapes differ.
- “Methane’s C–H bonds are strongly polar.” They’re nearly non-polar; and the symmetry cancels what polarity there is.
- “Ammonia forms as many hydrogen bonds as water.” It’s limited by having only one lone pair.
Key takeaways
- CH₄, NH₃ and H₂O each have four electron pairs in a tetrahedral arrangement and are sp³.
- Lone pairs (0, 1, 2) give tetrahedral, trigonal pyramidal and bent shapes with angles 109.5°, 107°, 104.5°.
- Methane is non-polar; ammonia and water are polar and form hydrogen bonds.
- Boiling points rise dramatically (−162 → −33 → 100 °C) despite nearly equal masses.
Practise with VSEPR practice questions.
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