What is Valence Bond Theory?
Valence bond (VB) theory explains covalent bonding through the overlap of atomic orbitals. A bond forms when two half-filled orbitals from different atoms overlap; stronger overlap creates stronger bonds. It accounts for bond directionality and distinguishes sigma (σ) and pi (π) bonds.
Valence bond theory states that covalent bonds form from the overlap of atomic orbitals, with electron density concentrated between nuclei. Stronger overlap = stronger bond; orbital geometry determines bond angles.
- 1↓Separate AtomsTwo H atoms, each with a 1s orbital containing one electron
- 2↓Orbital ApproachThe half-filled 1s orbitals come close; electron density begins to overlap
- 3↓Overlap MaximisedOrbitals achieve maximum overlap; electrons concentrate between the nuclei
- 4Bond FormedCovalent bond formed; nuclei held together by electron density
Step-by-step worked examples
Explain how a covalent bond forms between two hydrogen atoms using VB theory.
Each H atom has a half-filled 1s orbital with one electron As two H atoms approach, their 1s orbitals overlap Overlap creates electron density between nuclei Both electrons share the overlapping region; bond forms (H–H)
What is the difference between sigma (σ) and pi (π) bonds?
Sigma (σ) bonds: head-on overlap of orbitals along the internuclear axis Electron density concentrated directly between nuclei Pi (π) bonds: side-by-side overlap of p orbitals Electron density above and below the internuclear axis
Why does the O–H bond have a specific angle in water (104.5°)?
In H₂O, oxygen uses hybrid orbitals (sp³) Hybrid orbitals point toward tetrahedral positions Two O–H bonds form along two of these positions The angle between them is ≈104.5° (slightly less than 109.5° due to lone pair repulsion)
Flashcards
Quick quiz
Q1.Valence bond theory explains bonding through…
Q2.In a sigma (σ) bond, electron density is…
Q3.Stronger orbital overlap results in…
Q4.Pi (π) bonds result from…
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Common mistakes
Thinking all bonds have the same electron density pattern. — Correct: σ bonds concentrate density along the internuclear axis; π bonds concentrate it above/below.
Ignoring that overlap determines bond strength. — Correct: Better overlap = more electron density = stronger bond; this is central to VB theory.
Confusing VB theory with molecular orbital theory. — Correct: VB focuses on localized overlapping orbitals; MO theory treats electrons as delocalized across the molecule.
Assuming pi bonds are stronger than sigma bonds. — Correct: σ bonds are typically stronger due to better overlap; π bonds add extra bonding but are weaker individually.
FAQ
What is valence bond theory in one sentence?
Covalent bonds form when atomic orbitals from two atoms overlap, allowing electrons to occupy the overlapping region.
Why does VB theory predict bond angles?
The orientation of hybrid orbitals determines where bonding can occur; this sets the bond angles (e.g., 109.5° for tetrahedral sp³).
How does VB theory explain double bonds?
A double bond consists of one σ bond (head-on overlap) and one π bond (side-by-side overlap), keeping rotation restricted.
Is VB theory 100% accurate?
VB theory works well for molecules with clear, localized bonding but doesn't explain resonance or paramagnetism as well as MO theory.