Valence Bond Theory — Prelims Strategy

Prelims Strategy

For NEET prelims, questions on Valence Bond Theory are typically objective and require quick, accurate application of concepts. Here's a strategy:

1
Master Steric Number Calculation: — This is your fastest tool for hybridization. For any central atom, count the number of atoms directly bonded to it (which form sigma bonds) and add the number of lone pairs. Remember, a double or triple bond counts as only ONE sigma bond for steric number calculation. For example, in $CO_2$, carbon is bonded to two oxygens, so 2 sigma bonds. No lone pairs. Steric number = 2, hence $sp$ hybridization.
2
Know Hybridization-Geometry Correspondence: — Memorize the direct correlation: SN=2 $ightarrow sp$ (linear); SN=3 $ightarrow sp^2$ (trigonal planar); SN=4 $ightarrow sp^3$ (tetrahedral); SN=5 $ightarrow sp^3d$ (trigonal bipyramidal); SN=6 $ightarrow sp^3d^2$ (octahedral). This gives the electron geometry.
3
Apply VSEPR for Molecular Geometry: — Once hybridization (electron geometry) is known, use the number of lone pairs to determine the molecular geometry. Remember the order of repulsion: LP-LP > LP-BP > BP-BP, which causes bond angle distortions.
4
Practice Sigma and Pi Bond Counting: — For organic molecules, draw the full Lewis structure. Every single bond is 1 $sigma$. Every double bond is 1 $sigma$ + 1 $pi$. Every triple bond is 1 $sigma$ + 2 $pi$. Be systematic.
5
Understand s-character: — Remember that higher s-character ($sp > sp^2 > sp^3$) leads to shorter and stronger bonds. This helps in comparing bond lengths and strengths.
6
Beware of Trap Options: — Distractors often involve confusing electron geometry with molecular geometry, miscounting lone pairs, or incorrect hybridization assignments. Always double-check your steric number and lone pair count.
7
Practice with Ions: — For polyatomic ions, remember to adjust the total valence electrons for the charge before calculating lone pairs (add electrons for negative charge, subtract for positive charge).