What is the total number of electrons (both lone and bond pairs) in the Lewis structure of CO 3 2-?
a. 20 b. 22 c. 24 d. 28 e. 30
The correct answer and explanation is:
The correct answer is b. 22.
To determine the total number of electrons in the Lewis structure of the carbonate ion, CO₃²⁻, follow these steps:
- Count the valence electrons:
- Carbon (C) is in Group 14 of the periodic table and has 4 valence electrons.
- Oxygen (O) is in Group 16 and has 6 valence electrons each.
- The carbonate ion has a 2- charge, which means it has 2 extra electrons.
- Draw the basic structure:
In CO₃²⁻, carbon is the central atom and forms single bonds with three oxygen atoms. Each oxygen has lone pairs of electrons. The negative charge is distributed among the oxygens. - Distribute the electrons:
- Start by placing single bonds between carbon and each oxygen, which uses 6 electrons (3 bonds × 2 electrons).
- The remaining electrons are placed as lone pairs on the oxygen atoms. Initially, 18 electrons are left (24 total – 6 used for bonding).
- Each oxygen gets 6 electrons (3 lone pairs) to complete its octet, which uses 18 electrons (3 oxygens × 6 electrons).
- Account for the formal charge:
To minimize formal charges, one of the oxygen atoms forms a double bond with the carbon atom. This reduces the charge on that oxygen while leaving the other two oxygens with negative charges. The formal charges are balanced as:- One oxygen with a double bond to carbon has no formal charge.
- The two oxygens with single bonds each carry a -1 charge.
Thus, the total number of electrons in the structure is 22 (not 24). The extra 2 electrons from the charge are included in the lone pairs, which accounts for the 22 electrons in total.
In conclusion, the total number of electrons in the Lewis structure of CO₃²⁻ is 22.