VSEPR Theory — Prelims Strategy

Prelims Strategy

To effectively tackle VSEPR theory questions in NEET, a systematic approach is key. First, ensure you can quickly and accurately draw Lewis structures for various molecules and polyatomic ions. This is the absolute prerequisite.

Second, master the calculation of the steric number (SN) for the central atom. Remember the formula: $SN = \frac{1}{2} (\text{Valence electrons of central atom} + \text{Number of monovalent atoms} pm \text{Charge})$.

For divalent atoms like oxygen, they contribute zero to the 'monovalent atoms' count. Third, clearly distinguish between electron domain geometry (based on SN) and molecular geometry (based on SN and lone pairs).

Memorize the common geometries associated with different SN and lone pair combinations (e.g., SN=4, 0 LP = tetrahedral; SN=4, 1 LP = trigonal pyramidal; SN=4, 2 LP = bent). Fourth, understand the repulsion order (LP-LP > LP-BP > BP-BP) and its effect on bond angles.

This helps in comparing bond angles accurately. For numerical problems (like comparing bond angles), focus on the number of lone pairs. For conceptual questions (like identifying shapes or polarity), a quick sketch of the molecule's 3D structure can be very helpful.

Practice with a wide variety of examples, including noble gas compounds and polyatomic ions, as these are frequently tested. Be wary of trap options that confuse electron geometry with molecular geometry or ignore the effect of lone pairs.