The molecules NH 3 and BF 3 have the same general formula ( AB 3 ) but different shapes.

Find the shape of each of the above molecules.

Provide more examples of real molecules that have the same general formulas but different shapes.

(a)

Interpretation:

The shapes of the molecules, NH3 and BH3 are to be predicted.

Concept Introduction:

The VSEPR model is based on an important assumption according to which, “In a molecule, the surrounding atoms and electrons should arranged in way that the central atom should feel minimum repulsion from all of them”. The shape in VSEPR model gives specific angles for the corresponding shapes.

The steps for finding out the shape of NH3 molecule are as follows.

Step 1

In ammonia molecule, the central atom is nitrogen.

Step 2

The atomic number of nitrogen is seven. It has five electrons in its outer shell. Its Lewis structure is drawn as,

Figure 1

Here, central atom is nitrogen and it has one lone pair. The bond pair and lone pairs repel each other and it results in the pyramidal shape of the molecule. It is given as,

Figure 2

Hence, the molecule of ammonia is trigonal pyramidal in shape.

The shape of NH3 molecule is shown as,

Figure 3

According to the VSEPR theory, the trigonal pyramidal shape of NH3 molecule is formed by three bond pairs and one lone pair which means it has only one non-bonded electron. In case of ammonia molecule the bond angle is 107°.

The steps for finding out the shape of BF3 molecule are as follows

(b)

Interpretation:

The examples of the real molecules that have the same general formulas but have different shapes are to be stated.

Concept Introduction:

The VSEPR model is based on an important assumption according to which, “In a molecule, the surrounding atoms and electrons should arranged in way that the central atom should feel minimum repulsion from all of them”. The shape in VSEPR model gives specific angles for the corresponding shape.