Worked example 1
Assign the hybridization of each carbon in propene, CH₃-CH=CH₂, and rank the C-C bond angles.
Try it first: Count domains on each carbon separately.
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BondingHybridizationContent level: Challenge 20 min
Hybridization, Geometry and Bond Angles Together
What you'll be able to do: Combine hybridization, geometry and s character to explain and compare real bond angles.
Best after: Hybrid Orbitals from Domain Count
Introduction
The neat table breaks down in real molecules, and the deviations are themselves predictable.
These are recommended, not required. You can start this lesson at any time.
Learning objectives
- Rank bond angles using s character
- Assign hybridization to every atom in a multi-centre molecule
- Explain why PH₃ has smaller angles than NH₃
- Analyse molecules with perpendicular pi systems such as allene
- Use hybridization to justify observed geometry
Lesson
s character and angle
An sp hybrid is half s and half p, an sp² hybrid is one third s, and an sp³ hybrid is one quarter s. s orbitals are spherical and hold electron density closer to the nucleus, so more s character pulls the hybrids into a wider arrangement. That ordering gives 180, 120 and 109.5 degrees respectively, and it also explains why the C-H bond in ethyne is shorter and stronger than the one in ethane.
More s character also means higher electronegativity at that atom, which is why sp C-H bonds are weakly acidic.
When hybridization breaks down
Ammonia has angles near 107 degrees, consistent with sp³, but phosphine has angles near 93 degrees. Phosphorus is larger and its 3s and 3p orbitals differ more in energy and size, so mixing is poor. The bonds use nearly pure 3p orbitals, which are mutually perpendicular, and the lone pair stays in a mostly s orbital. Heavier group 15 and 16 hydrides show the same pattern.
Do not force sp³ onto every four-domain atom when the measured angle is near 90 degrees.
Multi-centre molecules
In allene, H₂C=C=CH₂, the central carbon has two domains and is sp, while the two terminal carbons have three domains each and are sp². The central sp carbon uses two perpendicular p orbitals for its two pi bonds, so the two CH₂ groups end up in planes rotated 90 degrees from one another and the molecule is not planar.
Key ideas
Rule
s character rule
Higher s character gives a wider bond angle, a shorter bond and a more electronegative atom.
Definition
s character
The fraction of s orbital in a hybrid: 50 percent for sp, 33 percent for sp², 25 percent for sp³.
Key concept
Phosphine
PH₃ has H-P-H angles near 93 degrees because the bonds are nearly pure 3p.
Key concept
Allene
sp central carbon and sp² terminal carbons give two perpendicular CH₂ planes.
Equation
s character
s fraction = 1 / (1 + number of p orbitals used)
Worked examples
Worked example 2
Explain why the H-P-H angle in PH₃ (about 93 degrees) is smaller than the H-N-H angle in NH₃ (about 107 degrees).
Try it first: Compare the size and energy match of the valence orbitals of N and P.
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Common mistakes
Assuming every AX₃E molecule has 107 degree angles.
Why it's wrong: Heavier central atoms hybridize poorly and approach 90 degrees.
Check instead: Consider the period of the central atom before quoting a value.
Assigning one hybridization to a whole molecule.
Why it's wrong: Hybridization is a property of each atom, and different atoms can differ.
Check instead: Count domains separately at every non-terminal atom.
Thinking allene is planar like ethene.
Why it's wrong: The central sp carbon uses two perpendicular p orbitals for two pi bonds.
Check instead: Track which p orbitals each pi bond uses.
Practice this skill
No practice questions are available for this topic yet. You can still practice the whole unit.
What you should now know
Hybridization, electron geometry and bond angle are three views of the same domain count. More s character in a hybrid orbital gives a wider bond angle, so sp (50 percent s) is wider than sp² (33 percent) which is wider than sp³ (25 percent). Heavy central atoms such as phosphorus use nearly pure p orbitals, which drives their angles toward 90 degrees.
- Hybridization, geometry and bond angle all follow the domain count
- More s character widens angles and shortens bonds
- Heavy central atoms hybridize poorly and approach 90 degree angles
- Each atom in a molecule gets its own hybridization label
- Perpendicular pi systems can twist a molecule out of plane
Sources and further reading
This lesson is original Chem Help content. No external sources were adapted.