Chapter U2.10, Problem 9E

Interpretation Introduction

Interpretation:

The tetrahedron shape of CH₄ molecule and the similarities with NH₃ and H₂O needs to be explained.

Concept introduction:

The Lewis structure of an organic compound represents the bonding of atoms with lone pairs (if any). It indicates the bonds with atoms and also the arrangement of atoms in a molecule.

Hybridization of an atom indicates the molecular geometry of a molecule. The formula to check the hybridization can be written as:

Hybridization = Number of sigma bonds + Number of lone pair

The molecular geometry of molecules with lone pair on central atom is given by the VSEPR theory.

Answer to Problem 9E

All these three molecules have the same type of hybridization and should have tetrahedral geometry but due to the presence of lone pairs on the central atom, the molecular geometry changes to a pyramidal and bent shape.

Explanation of Solution

For Lewis structure of CH₄ molecule, calculate total number of valence electrons:

CH₄ molecule = 4 electrons in C + 4 × 1 valence electrons in H = 8 electrons

Hybridization = Number of sigma bonds + Number of lone pair

Hybridization of C atom = 4 + 0 = 4 = sp³

Thus, with sp³ hybridization, the molecular shape must be tetrahedral and must be fit in a tetrahedron.

For Lewis structure of NH₃ molecule, calculate total number of valence electrons:

NH₃ molecule = 5 electrons in N + 3 × 1 valence electrons in H = 8 electrons

Hybridization = Number of sigma bonds + Number of lone pair

Hybridization of N atom = 3 + 1 = 4 = sp³

Thus, with sp³ hybridization, the molecular shape must be tetrahedral but due to the presence of lone pair on the central N atom, the geometry alters to trigonal pyramidal shape.

For Lewis structure of H₂O molecule, calculate total number of valence electrons:

H₂O molecule = 6 electrons in O + 1 × 2 valence electrons in H = 8 electrons

Hybridization = Number of sigma bonds + Number of lone pair

Hybridization of O atom = 2 + 2 = 4 = sp³

Thus, with sp³ hybridization, the molecular shape must be tetrahedral but due to the presence of two lone pairs on the central O atom, the geometry alters to a bent shape.

Conclusion

Although all these three molecules have same type of hybridization and should have tetrahedral geometry but due to presence of lone pairs on central atom, the molecular geometry changes to pyramidal and bent shape.