has bond angle = 104.5 deg.
has bond angle =107.8 deg.
Bond angle in general is inversely proportional to each of L, ENC and ENS.
L is the number of lone pairs,
ENC is the
electronegativity of the central atom.
ENS is the electronegativity of surrounding atom.
Double bonds and triple bonds do not affect
bond angles. Only single bonds do.
is same in both. L = 1 in NH3. And L = 2 in H2O. ENC is more for Oxygen. These factors result in the smaller bond
angle for water molecules than in Ammonia molecule.
In a Lone pair:
lp, the electrons are closer to central atom than in a bond pair: bp. Lp repels the neighbouring bonds more.
repulsion > lp-bp repulsion >
number of lone pairs in the central atom, more they repel the neighboring atoms. So more they cause the neighbor atoms to come
closer. So the bonds come together. So
the bond angle reduces.
Another reason we could give is that there is
a greater s-character in the N-H bonds (hybrid orbitals) in ammonia. In water molecules the O-H bonds (hybridized
orbitals) have less of s-character.
for clarity of application of above principles.
has resonant structure with one double bond and one single bond. So number of lone pairs is effectively less
So its bond angle is high 119 deg.
2. Methane CH4 has no lone pairs. It is tetrahedral. So it has a higher bond angle of 109.5 deg.
the other hand, NF3 and PF3 have a
different justification. Fluorine has a
high electro-negativity. So an electron pair in one F attracts an
electron pair in another F. So the bonds
come together as Fluorine atoms come together.
So bond angle is less than in
4. Between PF3 and PH3, the PF3 develops a
partial double bond due to resonance structures. So the effective number of lone pairs is
less. So the bond angle in PF3 is more than in PH3.
Between H2S and
H2O, Oxygen is more electro-negative.
But the bond angle in Water (104.5 deg) is more than in H2S (92.3 deg). Due to
formation of hybrid orbitals, the bonds should be at 90 deg. But Sulphur being bigger, it has more
electron pairs. There is more repulsion
against the Hydrogen atoms. Hydrogen
atoms in H2S have less positive charge on them than in water. So the Hydrogen atoms do not repel each other
as much. So the bonds are closer with
less bond angle.