Organic Chemistry · Structure and Bonding
Hybridization of Nitrogen, Oxygen, Phosphorus, and Sulfur
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In 30 seconds
Carbon is not the only atom that hybridizes — any atom that forms bonds and carries lone pairs can mix its valence orbitals, and the rule is exactly the same: count electron groups (σ bonds + lone pairs). Nitrogen in ammonia (NH3) and amines has three σ bonds plus one lone pair Nonbonding pair of valence electrons on an atom Full entry → = four groups, so it is sp³, trigonal pyramidal Shape from three bonds + one lone pair (NH3) Full entry → with a bond angle near 107°. Oxygen in water, alcohols, and ethers has two σ bonds plus two lone pairs = four groups, so it is also sp³, giving a bent shape with a 104.5° angle. Double-bonded (carbonyl) oxygen has only three groups and is sp². Phosphorus in phosphines and sulfur in thiols follow the same counting rules (sp³), although their measured angles (PH3 ≈ 93°, H2S ≈ 92°) reveal a real limitation of the hybridization model for third-row elements. In phosphates and sulfates, phosphorus and sulfur have no lone pairs and are sp³ tetrahedral.
Why this matters
Nitrogen and oxygen lone pairs point in specific directions, and that directionality controls much of the chemistry of life: how water hydrogen-bonds, how protein backbones fold, how DNA bases pair, and how amines act as bases and nucleophiles. Knowing whether a lone pair sits in an sp³, sp², or sp orbital tells you about its availability and its role in acid–base and addition chemistry. Phosphates make up the DNA backbone and ATP; thiols form the disulfide bonds that hold proteins together. Hybridization lets you predict all of these shapes before you ever meet the molecules in later chapters.
The college version
Core Concepts
Lone pairs count as electron groups
The electron-group count includes every σ bond and every lone pair; π bonds are not counted. Ammonia has only three N–H bonds, but its lone pair makes four groups — why nitrogen is sp³, not sp². Water's two bonds plus two lone pairs likewise give four. Forgetting lone pairs is the most common hybridization error.
Nitrogen: ammonia and amines (sp³, pyramidal)
Nitrogen in NH3 and in amines (R3N) uses four sp³ hybrid orbitals: three form σ bonds and one holds the lone pair. The shape is trigonal pyramidal — a tetrahedron with one corner occupied by the lone pair — with a bond angle near 107° rather than 109.5°, because the lone pair repels more strongly than a bond. That lone pair sits in a directional sp³ lobe, which is why amines are good bases and nucleophiles.
Oxygen: water, alcohols, and ethers (sp³, bent) vs. carbonyl oxygen (sp²)
Oxygen in water (H2O), alcohols (ROH), and ethers (ROR') has four electron groups (two σ bonds + two lone pairs) → sp³ with a bent shape; the lone pairs compress the angle from 109.5° to 104.5°. In a carbonyl group (C=O), oxygen has three groups (one σ bond + two lone pairs; π is not counted) → sp², lone pairs in the carbonyl plane. The same atom changes hybridization with its bonding situation.
Phosphorus: phosphines and phosphates
Phosphine (PH3) and phosphines (R3P) are counted like ammonia: three σ bonds + one lone pair → sp³, pyramidal. Phosphates (PO4³⁻) have phosphorus with four σ bonds and no lone pairs → sp³, tetrahedral, 109.5° — the geometry of the phosphate groups in DNA and ATP.
Sulfur: thiols, sulfides, and sulfates
Thiols (RSH) and sulfides (RSR') follow the oxygen pattern: two σ bonds + two lone pairs → sp³, bent. Sulfate (SO4²⁻) has sulfur with four σ bonds and no lone pairs → sp³, tetrahedral. Sulfur in SO2-type structures uses sp²; those cases are treated with resonance in later chapters.
Model limitations for third-row elements
The measured bond angles of PH3 (≈ 93°) and H2S (≈ 92°) sit far from the ideal 109.5° — close to the 90° angles of pure p orbitals. Third-row elements often bond with nearly unhybridized p orbitals, so the model is poorer for them than for carbon, nitrogen, or oxygen; the counting rule still works as bookkeeping, but expect angle deviations.
How It Works / Step-by-Step Process
- Draw the Lewis structure, including every lone pair.
- Count electron groups (σ bonds + lone pairs; π bonds do not count).
- Assign hybridization: 2 groups → sp; 3 → sp²; 4 → sp³.
- Predict the shape, expecting lone-pair compression (NH3 107°, H2O 104.5°).
- For third-row atoms (P, S), expect angles near 90° — the model has limits.
Common Confusions
| Do Not Confuse | With | The Difference |
|---|---|---|
| NH3 has 3 bonds → sp² | NH3 → sp³ | The lone pair is the fourth electron group |
| H2O has 2 bonds → sp² | H2O → sp³ | Two lone pairs + two bonds = four groups |
| All oxygen is sp³ | Carbonyl oxygen is sp² | The C=O oxygen has only 3 groups; the π bond is not counted |
| PH3/H2S angles ≈ 109.5° | Angles ≈ 93° / 92° | Third-row elements bond with mostly p orbitals; the model is approximate |
| P and S never exceed 4 groups | Expanded octets are possible | 5–6 groups give sp³d / sp³d² (a later topic, with model caveats) |
Eli explains
The same idea, in plain words
Explain it like I’m 10
Think of electron groups as people at a round table: every bond and every lone pair takes one seat. Nitrogen and oxygen bring invisible lone-pair guests who still claim a seat and push the visible guests (the bonds) closer together. That is why ammonia's angle is 107° and water's is 104.5°.
Worked example
Example 1: Hybridization table for common atoms
Count electron groups, then assign hybridization and shape:
| Species | σ bonds | Lone pairs | Electron groups | Hybridization | Shape (angle) |
|---|---|---|---|---|---|
| NH3 (N) | 3 | 1 | 4 | sp³ | trigonal pyramidal (≈ 107°) |
| H2O (O) | 2 | 2 | 4 | sp³ | bent (≈ 104.5°) |
| CH3OH (O) | 2 | 2 | 4 | sp³ | bent (≈ 104.5°) |
| CH2O (carbonyl O) | 1 | 2 | 3 | sp² | bent (≈ 120°) |
| PH3 (P) | 3 | 1 | 4 | sp³ | pyramidal (≈ 93° measured) |
| H2S (S) | 2 | 2 | 4 | sp³ | bent (≈ 92° measured) |
| PO4³⁻ (P) | 4 | 0 | 4 | sp³ | tetrahedral (109.5°) |
Water is the classic trap: two bonds but four groups — sp³, not sp².
Example 2: Amine vs. carbonyl oxygen — same atom, different hybridization
Compare methylamine (CH3NH2) with formaldehyde (CH2O). Nitrogen in methylamine: three σ bonds + one lone pair = four groups → sp³, trigonal pyramidal. The carbonyl oxygen of formaldehyde: one σ bond + two lone pairs = three groups → sp², lone pairs in the molecular plane. The counting rule alone answers both — no geometry needed in advance.
Example 3: Converting the O–H bond length of water (dimensional analysis)
Water's O–H bond is about 0.96 Å. Convert to picometers using 1 Å = 100 pm:
0.96 Å × 100 pm1 Å = 96 pm
And to nanometers using 1 nm = 1000 pm:
96 pm × 1 nm1000 pm = 0.096 nm
This is the bond hydrogen bonding repeatedly forms and breaks in water.
Key takeaways
- Lone pairs count as electron groups: NH3 = 4 groups → sp³ (pyramidal, ≈ 107°); H2O = 4 groups → sp³ (bent, ≈ 104.5°).
- Carbonyl oxygen (C=O) has 3 groups → sp²; alcohol/ether oxygen has 4 → sp³.
- Amines are sp³; the lone pair sits in an sp³ orbital — the basis of basicity and nucleophilicity.
- Phosphines (R3P) and thiols (RSH) follow the same counting rules → sp³.
- Phosphates (PO4³⁻) and sulfates (SO4²⁻): 4 σ bonds, no lone pairs → sp³ tetrahedral.
- Expect deviations for third-row elements (PH3 ≈ 93°, H2S ≈ 92°) — model limits.
Check yourself
6 review questions from the chapter. Try each one, then open the answer.
Why is nitrogen in ammonia sp³ even though it forms only three bonds?
Show answer
The lone pair counts as an electron group: three N–H σ bonds + one lone pair = four groups → sp³.
What shape does water have, and why is its angle less than 109.5°?
Show answer
Bent; the two lone pairs repel more strongly than bonds, compressing the angle from 109.5° to 104.5°.
What is the hybridization of the oxygen in a carbonyl group (C=O)?
Show answer
sp² — one σ bond + two lone pairs = three electron groups.
What is the hybridization and geometry of phosphorus in phosphate (PO4³⁻)?
Show answer
sp³, tetrahedral (four σ bonds, no lone pairs).
Why do PH3 and H2S deviate from the ideal tetrahedral angle?
Show answer
Third-row elements bond largely with unhybridized p orbitals; the measured angles (93°, 92°) sit near the 90° p-orbital angle — the model has limits for them.
What is the O–H bond length of water in picometers?
Show answer
About 96 pm (0.96 Å × 100 pm/Å).
Study tools & related lessonsKey vocabulary · Related
Key vocabulary
- lone pair
- Nonbonding pair of valence electrons on an atom
- trigonal pyramidal
- Shape from three bonds + one lone pair (NH3)
- bent geometry
- Shape from two bonds + two lone pairs (H2O)
- amine
- Compound R3N with an sp³ nitrogen
- carbonyl oxygen
- Double-bonded oxygen of a C=O group
- phosphine / thiol
- Phosphorus–hydrogen / sulfur–hydrogen compounds (R3P, RSH)
- electron group
- Any σ bond or lone pair around an atom
Sources & references
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