Chapter 8, Problem 73A

(a)

Interpretation Introduction

Interpretation: The error in the given electron dot structure of ${\text{CN}}^{-}$ needs to be determined. Also, the correct electron dot structure needs to be drawn.

Concept Introduction: An electron dot structure represents the arrangement of total valence electrons in a molecule. Here, electrons are represented as dots in pairs around the symbol of atoms of the molecule.

Explanation of Solution

In the given ion is ${\text{CN}}^{-}$ . The electron dot structure given has 2 bonding pairs of electrons between C and N atoms. Also, in the structure, there is one lone pair of electrons on the C atom is 1 and that on the N atom is 2. The given electron dot structure is incorrect because there is a triple bond between C and N atoms. Thus, there must be three bonding pairs of electrons.

The correct electron dot structure for ${\text{CN}}^{-}$ will be:

  

(b)

Interpretation Introduction

Interpretation: The error in the given electron dot structure of ${\text{PF}}_{\text{3}}$ needs to be determined. Also, the correct electron dot structure needs to be drawn.

Concept Introduction: An electron dot structure represents the arrangement of total valence electrons in a molecule. Here, electrons are represented as dots in pairs around the symbol of atoms of the molecule.

Explanation of Solution

The given molecule is ${\text{PF}}_{\text{3}}$. Here, the number of valence electrons in P and F atoms is 5 and 7 respectively. There are three F atoms thus, P forms single covalent bonds with each F atom. In the given electron dot, there is a double bond between one of the F and P atoms. This can not be possible as F cannot form a double bond with any other atom (required only one electro). The correct electron dot structure will be as follows: