Question

Below is an electron dot diagram drawn by a student. What was the student's error in creating this diagram

Answer 1

Answer:

The charges did not transfer from the cation to the anion to yield the correct subscripts

Explanation:

First of all, we see that we have $Al^{3+}$ and $O^{2-}$ in this ionic compound. Since this is a substance, it has a net charge of 0. This means the charge of a cation should be balanced by the charge of the anion.

The lowest common multiple of 3 and 2 is 6, meaning we need:

• \frac{6}{3}=2 $Al^{3+}$ cations;

• \frac{6}{2}=3 $O^{2-}$ anions;

Oxygen needs a total of 8 valence electrons, as it's the anion and it gains electrons to obtain an octet. We have 3 oxygen anions in total. Normally, oxygen has 6 electrons, so it needs to gain 2 electrons for each of the 3 oxygen atoms, that is, gain a total of 6 electrons. Those 6 electrons are donated by two aluminum atoms (each having 3 valence electrons), so two aluminum cations are balanced by three oxygen anions.

The Lewis diagram is correct, as it shows an octet on oxygen. We also have the correct charges. The subscripts, though, have to be 2 and 3 for Al and O respectively.