Compare Bond Angle of Molecules with Same Shape

The shape of a simple molecule can be deduced using the Valence Shell Electron Pair Repulsion (VSEPR) Theory.

There are 3 postulates used to determine the shape and bond angle.

1. Electron pairs around the central atom repel each other and stay as far apart as possible to minimise repulsion.

This gives us the basic shape of the molecule.

2. The repulsion of electron pairs decrease in the order: lone pair - lone pair > lone pair - bond pair > bond pair - bond pair

This gives us the actual shape and bond angle of the molecule.

Usually Postulates 1 and 2 are sufficient to determine the shape and bond angle.

Check out the detailed discussion on VSEPR and all the various shapes.

However to compare bond angles of 2 molecules with the exact same shape, Postulate 3, where we consider the difference in electronegativity, will be applicable.

Let's have 2 examples to illustrate.

1. NH₃ and PH₃

Both molecules have the same shape of trigonal pyramidal and bond angle of 107 degrees.

By comparing their electronegativity we can deduce the difference in their bond angles.

N is significantly more electronegative than H and can pull the bond pair of electrons closer to itself.

Since N is the central atom, the bond pair of electrons will be closer to each other and experience a greater repulsion.

Hence bond angle of NH₃ is larger.

P is slightly more electronegative than H so the bond pair of electrons will be further away from P.

The bond pair of electrons will experience weaker repulsion and can be closer together.

Hence bond angle of PH₃ is smaller.

2. NH₃ and NF₃

Again both molecules are trigonal pyramidal in shape.

This time the surrounding atoms are different but we can use the same concept to compare their bond angles.

In NH₃, N is more electronegative than H, bond pairs are closer to N and experience greater repulsion between them.

Hence bond angle of NH₃ is larger.

In NF₃, F is more electronegative than N, bond pairs are closer to surround atom F which means they are further away from central atom N.

The bond pairs will experience weaker repulsion hence bond angle of NF₃ is smaller.

Topic: Chemical Bonding, Physical Chemistry, A Level Chemistry, Singapore

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