Answer:
It's because it has 2 electrons on its outer ring
Step-by-step explanation:
It has a -2 ion due to it having 2 more electrons than protons making it an overall negative. It's well known in chemistry that the first ring can only hold 2 electrons while the second and furthermore can hold up to 8. But what happens when you have 9? a new ring starts and it would only contain 1 electron.. In our case here it has 2 more electrons than protons and those 2 electrons are located on the outer ring