Q: What is the correct formula for calculating the formal charge (FC) on an atom in a Lewis structure?

$$\text{FC} = \text{number of valence electrons} - (\text{number of lone pair electrons} + \frac{1}{2} \times \text{number of bonding electrons})$$

Question 1

Which of the following steps is NOT part of the standard procedure for drawing a Lewis structure for a polyatomic ion?

Accepted Answer

Assign lone pairs to the central atom first before the outer atoms.

Question 2

What is the correct formula for calculating the formal charge (FC) on an atom in a Lewis structure?

Accepted Answer

$$	ext{FC} = 	ext{number of valence electrons} - (	ext{number of lone pair electrons} + \frac{1}{2} 	imes 	ext{number of bonding electrons})$$

Question 3

Which of the following statements about resonance structures is TRUE?

Accepted Answer

The actual molecule is best represented as a weighted average of all resonance structures.

Question 4

For the carbonate ion ($$\mathrm{CO_3$^{2-}}$$), how many total valence electrons must be accounted for when drawing its Lewis structure?

Accepted Answer

$$24$$

Question 5

Which atom is most likely to be the central atom in a Lewis structure?

Accepted Answer

The atom with the lowest electronegativity (excluding hydrogen and halogens).

Question 6

Which of the following molecules or ions is most likely to have an incomplete octet in its Lewis structure?

Accepted Answer

$$\mathrm{BF_3$}$$

Question 7

Which of the following elements can have an expanded octet in a Lewis structure?

Accepted Answer

Sulfur

Question 8

In a valid Lewis structure for $$\mathrm{CO_2$}$$, what is the formal charge on each oxygen atom?

Accepted Answer

$$0$$

Question 9

Which of the following is the best resonance structure for a molecule?

Accepted Answer

The structure with the most atoms having formal charges of zero.

Question 10

A student draws a Lewis structure for $$\mathrm{NO_3$^-}$$ and calculates the formal charges as follows: N = $$+1$$, one O = $$-1$$, two O = $$0. $$What is the sum of the formal charges, and is this consistent with the ion's charge?

Accepted Answer

$$-1$$; consistent with the ion's charge

Question 11

Which of the following is a correct statement about the placement of formal charges in resonance structures?

Accepted Answer

Negative formal charges should be placed on the most electronegative atom.

Question 12

Which of the following molecules or ions is most likely to have more than four bonds to its central atom in a valid Lewis structure?

Accepted Answer

$$\mathrm{$$PCl_5$$}$$

Question 13

Why is hydrogen never the central atom in a Lewis structure?

Accepted Answer

Hydrogen can only form one bond and cannot accommodate more than two electrons.

Question 14

Which of the following best explains why $$\mathrm{SO_4$^{2-}}$$ can have an expanded octet on the sulfur atom?

Accepted Answer

Sulfur is in period 3 and has available d orbitals for bonding.

Question 15

A Lewis structure for $$\mathrm{BF_3$}$$ shows boron with only six electrons around it. What is this an example of?

Accepted Answer

Incomplete octet

Question 16

Which of the following is a real-world consequence of incorrectly assigning formal charges in a Lewis structure for a pharmaceutical molecule?

Accepted Answer

The molecule may be predicted to have incorrect reactivity or binding properties.

Question 17

Which of the following ions or molecules is most likely to have resonance structures?

Accepted Answer

$$\mathrm{CO_3$^{2-}}$$

Question 18

When drawing the Lewis structure for $$\mathrm{CO_2$}$$, what is the total number of electrons that must be represented?

Accepted Answer

$$16$$

Question 19

Which of the following is a correct statement about the sum of formal charges in a neutral molecule?

Accepted Answer

The sum of the formal charges must be zero.

Lewis Structures and Formal Charge: Lecture 13

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