Problem 80 - Chemistry: An Atoms-Focused Approach

Question

Some medicines used in treating depression contain lithium carbonate. Draw the Lewis structure for \(\mathrm{Li}_{2} \mathrm{CO}_{3}\).

Step-by-Step Solution

Answer

Answer: The Lewis structure for Li₂CO₃ is: Li-O-C-O-Li | O The carbonate ion (CO₃²⁻) has a charge of -2, and to balance this, there are two Li atoms with charges of +1 each. Carbon and oxygen atoms form the central carbonate ion, while Li atoms bond to the oxygen atoms of the carbonate ion.

Step 1: Calculate the total number of valence electrons

The elements in \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) have the following valence electrons: Li has 1, C has 4, and O has 6. Since there are two Li atoms, our total count of valence electrons is: \((1 \times 2) + 4 + (6 \times 3) = 2 + 4 + 18 = 24\).

Step 2: Arrange the atoms in a skeleton structure

The central atom in this structure should be the one with the most bonding preferences, which is Carbon. So we will arrange the atoms like this: C is in the center with an O atom connected to it on each side (a total of 3 O atoms), and each O atom will be connected to a Li atom. In this arrangement, the carbonate ion (\(\mathrm{CO}_{3}^{2-}\)) will be formed first before bonding to the Li atoms.

Step 3: Assign electrons to create bonds

For each bond between C and O, we need two electrons. So, we need a total of \(2 \times 3 = 6\) electrons to create bonds between the central C atom and the three O atoms. We have used 6 out of 24 valence electrons.

Step 4: Distribute the remaining electrons to satisfy the octet rule

There are 18 valence electrons left. First, we will satisfy the octet rule for the three O atoms. Each O atom needs 6 more electrons to complete the octet, totaling 18 electrons. After this step, all O atoms have a full octet. Since Li is in Group 1 and has only 1 valence electron, it wants to lose 1 electron to become stable. Carbon already has its full octet from the bonds with oxygen atoms.

Step 5: Assign charges and combine ions to form the final Lewis structure

The carbonate ion (\(\mathrm{CO}_{3}^{2-}\)) has a charge of -2 due to the extra electrons around the oxygen atoms. To balance this, we need two Li atoms with charges of +1 each, as they lose their valence electrons. Thus, we combine \(\mathrm{CO}_{3}^{2-}\) with two \(\mathrm{Li}^{+}\) ions to form the final Lewis structure for \(\mathrm{Li}_{2} \mathrm{CO}_{3}\). The Lewis structure for \(\mathrm{Li}_{2} \mathrm{CO}_{3}\) is below: \(\mathrm{Li-O-C-O-Li}\) \(\,\,\,\,\,\,\,\,\,\,\,\, \\ |\,\) \(\,\,\,\,\,\,\,\,\,\,\,\, \\O\) With the Li atoms having +1 charges and the carbonate ion having a -2 charge.