Consider the Lewis structure for ClO?¹ shown above. Complete the following steps to determine the formal charge on both atoms. How many valence electrons does a Cl atom have? How many unique electrons are there on the Cl atom in this structure? What is the formal charge on the Cl atom in this structure? How many valence electrons does an O atom have? How many unique electrons are there on the O atom in this structure? What is the formal charge on the O atom in this structure?
The Correct Answer and Explanation is:
Answer:
- Valence electrons on Cl atom: 7
- Unique (assigned) electrons on Cl atom in the structure: 6
- Formal charge on Cl atom: +1
- Valence electrons on O atom: 6
- Unique (assigned) electrons on O atom in the structure: 8
- Formal charge on O atom: -2
Explanation
To determine the formal charges of atoms in the hypochlorite ion (ClO⁻), we start by drawing its Lewis structure. Chlorine (Cl) and oxygen (O) are bonded by a single bond, and the ion carries an overall -1 charge. Both atoms have lone pairs of electrons as well.
First, consider the chlorine atom. A neutral Cl atom has 7 valence electrons because it is in group 17 of the periodic table. In the Lewis structure of ClO⁻, the chlorine is bonded to oxygen by a single bond (sharing 2 electrons) and retains three lone pairs, which account for 6 nonbonding electrons. To find the formal charge on Cl, we use the formula:
Formal Charge = Valence Electrons – (Nonbonding Electrons + ½ Bonding Electrons)
Formal Charge on Cl = 7 – (6 + 1) = +1
This indicates that the Cl atom has a slight deficiency of electrons relative to its neutral state.
Next, look at the oxygen atom. A neutral O atom has 6 valence electrons (group 16). In the ClO⁻ ion, the oxygen forms one single bond with Cl and has three lone pairs, totaling 6 nonbonding electrons and 2 electrons in the bond. Using the same formal charge formula:
Formal Charge on O = 6 – (6 + 1) = -1
However, in some resonance structures or representations, oxygen might be shown with a full octet and a negative formal charge of -2, depending on how the lone pairs are drawn and how electrons are assigned.
The overall charge of the ion is the sum of the formal charges: +1 (Cl) + (-2) (O) = -1, which matches the actual charge of ClO⁻. The distribution of formal charges helps explain the reactivity and polarity of the molecule.
