Angular Shape of Molecules

Introduction

Shapes of molecules are an important aspect when we study molecules in chemistry. Studying shapes and geometry gives us an insight into bonding, steric hindrance, bond angle, and bond strength. All these factors can be easily predicted correctly by studying the shapes and structures of the molecules. The angular shape in molecules are common because a lot of molecules have an imperfect angle in between them, so they form angular shapes.

Different shapes of molecules

The shape of a compound is determined by the number of electrons around the central atom, the number of bonds, and the nature of these pairs, like a bonding pair or lone pair.

The shape of the molecule is decided by a theory known as VSEPR theory (valence shell electron pair repulsion).

The VSEPR proposition gives the figure of simple notes, but theoretically, it doesn’t explain them, and also it has limited operations. To overcome these limitations, the two important propositions grounded on quantum mechanical principles are introduced. These are Valence Bond (VB) propositions and Molecular Orbital (MO) propositions.

The main learning from VSEPR theory is as follows:

• Electron pairs are always repelling each other and try to remain apart from each other.
• Different electron pairs have different orders of repulsion, and the order is:

[lone pair-lone pair > lone pair-bond pair > bond pair-bond pair]

• If there are zero lone pairs, then the shape and geometry are the same; otherwise, they are different.
• Geometry depends on both lone and bond pairs, whereas shape depends only on bond pairs.
• The steric number is defined as lone pair + bond pair.

Angular shape of molecules

Angular shapes of molecules are also referred to as V-shapes or bent shapes.

• The compounds with steric number 3:

The compounds with 0 lone pairs and 3 bond pairs are trigonal in shape.

The compounds with 1 lone pair and 2 bond pairs are bent in shape.

For example, SO2 is an sp2 compound with a bond angle of around 120 degrees between two oxygen bonds. Theoretically, the shape should have been triangular planar, but honestly, it is found to be bent or v-shaped. The reason being the lone pair-bond pair repulsion is much more as compared to the bond pair-bond pair repulsion. So the angle is reduced to 119.5° from 120°. 

• The compounds with steric number 4:

The compounds with 0 lone pairs and 4 bond pairs are tetrahedral in shape.

The compounds with 1 lone pair and 3 bond pairs are in a trigonal pyramidal shape.

The compounds with 2 lone pairs and 2 bond pairs are bent or v-shape.

For example, H2O is an sp3 compound with a bond angle of around 104.5 degrees between 2 hydrogen bonds. The shape would have been tetrahedral if there were all bp; however, since only two lp are present, the shape is distorted tetrahedral or angular. This is because lp-lp repulsion is higher than lp-bp repulsion, which is again higher than bp-bp repulsion. Thus, the angle is reduced from 109.5° to 104.5°.

T shaped molecule

The compounds with steric number 5:

The compounds with 0 lone pairs and 5 bond pairs are of trigonal bipyramidal shape.

The compounds with 1 lone pair and 4 bond pairs are see-saw shaped.

The compounds with 2 lone pairs and 3 bond pairs are t-shaped.

For example, ClF3 is an sp3d compound with a bond angle of around 90 degrees between fluorine bonds. The lone pair is at the equatorial position, so there are fewer lone pair – bond pair (lp-bp) repulsions as compared to others in which the lone pairs are at axial positions. So the structure is most stable.

Benefits of learning about shapes and geometry

Knowing the shape and geometry of the molecule can help determine the polarity, reactivity, phase of matter, colour, magnetism, as well as biological activity. One can also figure out the bond angle, stability, hybridization, steric hindrance, lattice enthalpy, etc., by knowing the shapes and geometry of a molecule. 

Conclusion

The VSEPR model used for predicting the geometrical shapes of molecules is based on the assumption that electron pairs repel each other and they tend to remain as far apart as possible. Electron pairs are always repelling each other and try to remain apart from each other. Different electron pairs have different orders of repulsion, and the order is:

[lone pair-lone pair > lone pair-bond pair > bond pair-bond pair]. If there are zero lone pairs, then the shape and geometry are the same; otherwise, they are different. 

Geometry depends on lone pair and bond pair both, whereas shape depends on bond pair only. The steric number is defined as lone pair + bond pair. The compounds with 1 lone pair and 2 bond pairs are bent in shape—for example, SO2. The compounds with 2 lone pairs and 2 bond pairs are bent or v-shape—for example, H2O. The compounds with 2 lone pairs and 3 bond pairs are t-shaped—for example, ClF3.