Chapter U2, Problem C7.3RE

Interpretation Introduction

Interpretation : Factors determining a shape of molecule are to be described.

Concept Introduction : The bonded electron pairs occupy space between the atoms in a molecule. The space occupied by the electrons is called an electron domain. It may consist of bonded pairs, lone pairs or multiple bonded pairs of electrons.

Answer to Problem C7.3RE

The shape of a molecule is affected by following:

• Bonded and lone pair of electrons.
• Thelocation and number of its electron domains.

Explanation of Solution

Bonded and lone pair of electronsaffects the shape of a molecule. Lewis dot structures help in knowing the number of electron domains in a molecule. The final shape of a molecule is determined by the fact that electron domains are located as far apart from one another as possible.The shape of water is bent due to two lone pairs of electrons on oxygen atom. So there are two bonded pairs and two lone pairs. The four pairs are arranged in tetrahedral shape. This results in bent shape. Ammonia has a pyramidal shape due to a lone pair on nitrogen. Methane has a tetrahedral shape due to four electron domains.

The location and number of electron domains in a molecule determine its shape. An atom having a double or triple bond will have fewer electron domains surrounding it. This gives molecules planar and linear shapes. For example, the molecular shape of ethane is trigonal planar and shape of ethyne is linear. Large molecules may have different types of geometric shapes within them. Areas around double bonds are flat and those around triple bonds are linear. For example, in heptanoic acid, C₇H₁₄O₂, the different electron domains cause the molecule to have a zigzag shape.

Conclusion

Molecules have a three dimensional shape. Some molecules are flat, some are linear, some are trigonal and some are pyramidal in shape. The valence electrons determine the ultimate shape of a molecule.