Bond Length and Bond Angle — Prelims Strategy

Prelims Strategy

To effectively tackle NEET questions on bond length and bond angle, a systematic approach is crucial. For conceptual questions on bond angles, always start by identifying the central atom. Then, determine its hybridization and count the number of lone pairs.

Apply VSEPR theory, remembering the repulsion order: Lone Pair-Lone Pair (LP-LP) > Lone Pair-Bond Pair (LP-BP) > Bond Pair-Bond Pair (BP-BP). This will help you predict deviations from ideal bond angles.

For example, $sp^3$ hybridized atoms without lone pairs have $109.5^\circ$, but with one lone pair (e.g., NH$_3$), it's $107^circ$, and with two lone pairs (e.g., H$_2$O), it's $104.5^circ$. Also, consider the electronegativity of surrounding atoms: more electronegative surrounding atoms pull electron density away from the central atom, reducing BP-BP repulsion and slightly decreasing the bond angle.

For bond length questions, focus on bond order first: triple bonds are shortest, then double, then single. Next, consider hybridization: greater s-character leads to shorter bonds (e.g., $sp < sp^2 < sp^3$ for C-H bond lengths).

Don't forget resonance structures, where bond lengths are averaged (e.g., C-O bonds in CO$_3^{2-}$ are intermediate between single and double). Atomic size is also a direct factor: larger atoms form longer bonds.

Practice comparing series of molecules (e.g., C-F vs C-Cl bond lengths) and applying these rules. Be wary of trap options that reverse the trends or ignore lone pair effects. Always draw Lewis structures if unsure to correctly identify lone pairs and bonding domains.