Question

Question: Calculate the formal charges on the atoms shown in red.

Short answer

Answer

The formal charge on O is +1.

The formal charge on B is -1.

Step-by-step solution

Formal charge

Formal charge means the electric charge present on an atom in a molecule. It indicates the magnitude and whether a charge is positive or negative.

Calculation of formal charge

The formal charge on an atom is equal to the number of valence electrons in a neutral and isolated atom minus the number of electrons present in its bonded atom form in a molecule.

The number of electrons in a bonded atom is calculated as half the number of bonding electrons plus the nonbonding, that is, lone-pair electrons.

$$\begin{gathered} \text{formalcharge=}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{freeatom}\end{array}\right)\text{-}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{bondedatom}\end{array}\right) \\ \text{=}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{freeatom}\end{array}\right)\text{-}\left(\frac{\text{numberofbondingelectro}n}{2}\text{+numberofnonbondingelectron}\right) \end{gathered}$$

Each bond formed has two electrons in it.

Calculation of formal charge on O:

Structure showing O linked with two CH₃ and one B.

Using the above formula:

$$\begin{gathered} \text{formalcharge=}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{freeatom}\end{array}\right)\text{-}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{bondedatom}\end{array}\right) \\ \text{=}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{freeatom}\end{array}\right)\text{-}\left(\frac{\text{numberofbondingelectro}n}{2}\text{+numberofnonbondingelectron}\right) \\ \text{=6-}\left(\frac{6}{2}\text{+2}\right) \\ \text{=6-5} \\ =1 \end{gathered}$$

Calculation of formal charge on B:

Using the above formula:

$$\begin{gathered} \text{formalcharge=}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{freeatom}\end{array}\right)\text{-}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{bondedatom}\end{array}\right) \\ \text{=}\left(\begin{array}{c}\text{numberof}\\ v\text{alanceelectronin}\\ \text{freeatom}\end{array}\right)\text{-}\left(\frac{\text{numberofbondingelectro}n}{2}\text{+numberofnonbondingelectron}\right) \\ \text{=3-}\left(\frac{8}{2}\text{+0}\right) \\ \text{=3-4} \\ =-1 \end{gathered}$$