Valence Bond Theory — Revision Notes

⚡ 30-Second Revision

VBT Core: — Covalent bond by overlap of half-filled atomic orbitals with paired electrons.
Hybridization: — Mixing of atomic orbitals to form new, degenerate hybrid orbitals for effective bonding.
Types of Hybridization:

- $sp$: Linear, $180^circ$, 2 hybrid orbitals (1s + 1p) - $sp^2$: Trigonal Planar, $120^circ$, 3 hybrid orbitals (1s + 2p) - $sp^3$: Tetrahedral, $109.5^circ$, 4 hybrid orbitals (1s + 3p) - $sp^3d$: Trigonal Bipyramidal, 5 hybrid orbitals (1s + 3p + 1d) - $sp^3d^2$: Octahedral, 6 hybrid orbitals (1s + 3p + 2d)

Steric Number (SN): — SN = ($sigma$ bonds) + (Lone Pairs). Directly gives hybridization.
Sigma ($sigma$) Bond: — Head-on overlap (s-s, s-p, p-p axial). Strong, free rotation.
Pi ($pi$) Bond: — Lateral overlap (p-p sideways). Weaker, restricted rotation.
Bond Counting: — Single bond = $1sigma$; Double bond = $1sigma + 1pi$; Triple bond = $1sigma + 2pi$.
s-character: — Higher s-character $implies$ shorter, stronger bond ($sp > sp^2 > sp^3$).
Lone Pair Effect: — LP-LP > LP-BP > BP-BP repulsion $implies$ distorts bond angles.

2-Minute Revision

Valence Bond Theory (VBT) explains covalent bonding through the overlap of atomic orbitals, each containing an unpaired electron. This overlap leads to electron pairing and bond formation. A key concept is hybridization, where atomic orbitals (s, p, d) on a central atom mix to form new, equivalent hybrid orbitals ($sp, sp^2, sp^3, sp^3d, sp^3d^2$).

The number of hybrid orbitals formed equals the number of atomic orbitals mixed. The steric number (number of sigma bonds + number of lone pairs) quickly determines hybridization and thus the electron geometry.

For example, a steric number of 4 implies $sp^3$ hybridization and a tetrahedral electron geometry. Bonds can be **sigma ($sigma$) (formed by head-on overlap, strong, free rotation) or pi ($pi$)** (formed by lateral overlap of unhybridized p-orbitals, weaker, restricted rotation).

A single bond is $1sigma$, a double bond is $1sigma + 1pi$, and a triple bond is $1sigma + 2pi$. The s-character in hybrid orbitals influences bond properties: higher s-character leads to shorter and stronger bonds.

Lone pairs on the central atom exert greater repulsive forces than bond pairs, causing deviations from ideal bond angles and influencing the molecular geometry (e.g., bent water from tetrahedral electron geometry).

5-Minute Revision

Valence Bond Theory (VBT) is a fundamental model for understanding covalent bond formation. It postulates that a covalent bond arises from the overlap of half-filled atomic orbitals, each containing an unpaired electron, with opposite spins.

The greater the extent of this overlap, the stronger the bond. To explain observed molecular geometries and bond equivalences, VBT introduces hybridization, a theoretical mixing of atomic orbitals (s, p, d) on a central atom to form a new set of degenerate hybrid orbitals.

For instance, carbon in methane undergoes $sp^3$ hybridization, forming four equivalent $sp^3$ orbitals that point towards the corners of a tetrahedron, explaining its $109.5^circ$ bond angles.

Hybridization types are directly linked to the steric number (SN), which is the sum of sigma bonds and lone pairs around the central atom:

SN=2: $sp$ (linear, $180^circ$)
SN=3: $sp^2$ (trigonal planar, $120^circ$)
SN=4: $sp^3$ (tetrahedral, $109.5^circ$)
SN=5: $sp^3d$ (trigonal bipyramidal)
SN=6: $sp^3d^2$ (octahedral)

Covalent bonds are categorized into **sigma ($sigma$) and pi ($pi$)** bonds. Sigma bonds result from head-on (axial) overlap of orbitals (s-s, s-p, p-p axial) and are strong, allowing free rotation.

Pi bonds form from lateral (sideways) overlap of unhybridized p-orbitals, are weaker, and restrict rotation. A single bond is $1sigma$, a double bond is $1sigma + 1pi$, and a triple bond is $1sigma + 2pi$.

For example, in ethene ($C_2H_4$), each carbon is $sp^2$ hybridized, forming three $sigma$ bonds (two C-H, one C-C) and one $pi$ bond (C-C).

The s-character of hybrid orbitals significantly impacts bond properties. Higher s-character (e.g., $sp$ with 50%) means electrons are closer to the nucleus, leading to shorter and stronger bonds compared to $sp^2$ (33.

3%) or $sp^3$ (25%). Finally, lone pairs on the central atom occupy hybrid orbitals and exert greater repulsive forces than bonding pairs (LP-LP > LP-BP > BP-BP), compressing bond angles and distorting the molecular geometry from the ideal electron geometry (e.

g., water's bent shape from $sp^3$ hybridization). While powerful, VBT has limitations, particularly for delocalized systems and magnetic properties, where Molecular Orbital Theory offers a more complete description.

Prelims Revision Notes

Valence Bond Theory (VBT) - NEET Revision Notes

1. Core Principle: Covalent bond forms by overlap of half-filled atomic orbitals with opposite spin electrons.

2. Hybridization:

* Mixing of atomic orbitals (s, p, d) on a central atom to form new, equivalent hybrid orbitals. * Purpose: Explains observed molecular geometries and bond equivalences. * Number of hybrid orbitals = Number of atomic orbitals mixed.

3. Steric Number (SN) Method for Hybridization:

* SN = (Number of sigma ($sigma$) bonds) + (Number of lone pairs on central atom). * SN = 2: $sp$ hybridization (e.g., $BeCl_2, C_2H_2, CO_2$). Electron Geometry: Linear. Molecular Geometry: Linear.

Bond Angle: $180^circ$. * SN = 3: $sp^2$ hybridization (e.g., $BF_3, C_2H_4, SO_2$). Electron Geometry: Trigonal Planar. * 0 LP: Trigonal Planar ($BF_3$), $120^circ$. * 1 LP: Bent ($SO_2$), $<120^circ$.

* SN = 4: $sp^3$ hybridization (e.g., $CH_4, NH_3, H_2O$). Electron Geometry: Tetrahedral. * 0 LP: Tetrahedral ($CH_4$), $109.5^circ$. * 1 LP: Trigonal Pyramidal ($NH_3$), $107^circ$. * 2 LP: Bent ($H_2O$), $104.

5^circ$. * **SN = 5:**$sp^3d$hybridization (e.g.,$PCl_5, SF_4, ClF_3, I_3^-$). Electron Geometry: Trigonal Bipyramidal. * 0 LP: Trigonal Bipyramidal ($PCl_5$). * 1 LP: Seesaw ($SF_4$). * 2 LP: T-shaped ($ClF_3$).

* 3 LP: Linear ($I_3^-$). * SN = 6: $sp^3d^2$ hybridization (e.g., $SF_6, XeF_4, ICl_4^-$). Electron Geometry: Octahedral. * 0 LP: Octahedral ($SF_6$). * 1 LP: Square Pyramidal ($BrF_5$). * 2 LP: Square Planar ($XeF_4, ICl_4^-$).

4. Sigma ($sigma$) and Pi ($pi$) Bonds:

* **Sigma ($sigma$) Bond:** Formed by head-on (axial) overlap (s-s, s-p, p-p axial). Strongest, allows free rotation. * **Pi ($pi$) Bond:** Formed by lateral (sideways) overlap of unhybridized p-orbitals. Weaker, restricts rotation. * Bond Counting Rules: * Single bond = $1sigma$ * Double bond = $1sigma + 1pi$ * Triple bond = $1sigma + 2pi$

5. Effect of s-character on Bond Properties:

* % s-character: $sp (50$. * Higher s-character $implies$ electrons closer to nucleus $implies$ shorter bond length, stronger bond, more electronegative hybrid orbital. * Order of C-C bond length: $sp < sp^2 < sp^3$. * Order of C-C bond strength: $sp > sp^2 > sp^3$.

6. Lone Pair Repulsion:

* Order of repulsion: Lone Pair-Lone Pair (LP-LP) > Lone Pair-Bond Pair (LP-BP) > Bond Pair-Bond Pair (BP-BP). * Lone pairs cause distortion of ideal bond angles (e.g., $NH_3$ $107^circ$ vs. $CH_4$ $109.5^circ$).

7. Limitations of VBT:

* Fails to explain paramagnetism of $O_2$. * Cannot explain electron delocalization in resonance structures (e.g., benzene) without resonance concept. * Does not explain electron-deficient molecules (e.g., $B_2H_6$).

Quick Tip: For polyatomic ions, remember to add/subtract electrons for negative/positive charges when calculating total valence electrons for lone pair determination.

Vyyuha Quick Recall

Hybridization Shapes Geometry Bonds Lone Pairs:

Hybridization: $sp, sp^2, sp^3, sp^3d, sp^3d^2$ (from SN).
Steric Number: $sigma$ bonds + Lone Pairs.
Geometry: Linear, Trigonal Planar, Tetrahedral, TBP, Octahedral (electron geometry).
Bonds: $sigma$ (head-on), $pi$ (sideways).
Lone Pairs: Distort angles (LP-LP > LP-BP > BP-BP).