Which has lowest value of bond Angle?

Question

Accepted Answer

The correct option is (3):

Water ($H_2O$) has the lowest bond angle among the given options due to the presence of two lone pairs of electrons on the central oxygen atom. According to VSEPR (Valence Shell Electron Pair Repulsion) theory, electron pairs around a central atom repel each other and arrange themselves to minimize this repulsion. While all four molecules listed ($CH_4$, $NH_3$, $H_2O$) in options 1, 2, and 3, have a central atom with four electron domains (i.e., they are $sp^3$ hybridized and have a tetrahedral electron geometry), the number of lone pairs differs. Methane ($CH_4$) has no lone pairs, ammonia ($NH_3$) has one lone pair, and water ($H_2O$) has two lone pairs. Lone pair-lone pair repulsion is greater than lone pair-bond pair repulsion, which is greater than bond pair-bond pair repulsion. As the number of lone pairs increases, the repulsion between them forces the bond pairs closer together, thus decreasing the bond angle. Methane, with no lone pairs, has an ideal tetrahedral bond angle of 109.5°. Ammonia, with one lone pair, has its bond angle reduced to approximately 107°. Water, with two lone pairs, experiences even stronger repulsion, compressing its bond angle further to about 104.5°.

The incorrect option is (1):

Methane ($CH_4$) has a tetrahedral molecular geometry with no lone pairs on the central carbon atom. This results in an ideal bond angle of 109.5°, which is significantly larger than that of water.

The incorrect option is (2):

Ammonia ($NH_3$) has a trigonal pyramidal molecular geometry with one lone pair on the central nitrogen atom. This lone pair causes a repulsion that reduces the bond angle from the ideal 109.5° to approximately 107°, which is still greater than the bond angle in water.

The incorrect option is (4):

Carbon dioxide ($CO_2$) has a linear molecular geometry. The central carbon atom is double-bonded to two oxygen atoms with no lone pairs. This arrangement leads to a bond angle of 180°, which is the largest among all the given options.