Question

The smaller the electronegativity of
the central atom, the smaller is
the
Answer
A.
bond energy
B.Bond angle
C. high distortion
D. bond length
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Answer 1

Answer:

BOND ANGLE

Explanation:

The electronegativity of the central atom does not really influence bond angles in any meaningful way. The size of atoms and the orbitals used and useable are much more important.

Sometimes, students may be taught concepts such as electronegativity or ‘lone pairs require space’ to explain why H2O has a smaller bond angle (105∘) than NH3 (107∘) or similar. However, careful analysis of the molecular orbitals reveals that sterics are a much more important reason than ‘lone pairs requiring more space and thus making tetrahedral sp3 contract’. Indeed, as soon as a lone pair exists on a central atom, there is a tendency to have the lone pair occupy an orbital with an s-contribution as high as possible and to have bonding orbitals with a maximised p-contribution. p-orbitals, having a direction, can form stronger σ bonds due to better overlap.

This is why the bond angle drops quickly when comparing second-period to third-period central elements to close to 90∘ but then hardly changes down the group. On the other hand, the reduced bond angles observed from ammonia to water are mainly due to more steric stress on nitrogen — which needs to accomodate an additional hydrogen and thus needs to space them out more.

Indeed, if you compare the bond angles of different OX2 compounds, the only general trend is ‘the larger the second element’s atoms, the greater the bond angle.’ See the table below (all values taken from Wikipedia).

XHFClBr∠(X−O−X)104.5∘103∘110.9∘112∘∠(X−N−X)107.8∘102.5∘−/−−/−

The drop in bond angles from hydrogen to fluorine is sufficiently explained by the much longer bond lengths of the OF2/NF3 species when compared to the corresponding hydrogen species due to fluorine using p-orbitals for bonding where hydrogen uses its 1s orbital.

While the bond lengths also increase when going from fluorine to chlorine and bromine, the angle also does. Note especially OBr2 whose bond angle is larger than the tetrahedral bond angle. Any argument involving electronegativity should assume a smaller angle here since the electronegativity of the partners drops from fluorine to bromine.