Why does oxygen form the #"O"_2^(2-)# ion?

1 Answer
Mar 9, 2015

Oxygen needs 2 more electrons to fill its outermost shell.

If you look in the periodic table, you'll notice that oxygen has an atomic number equal to 8. This means that the neutral oxygen atom has 8 electrons.

Here's a very, very cool picture of a model representing the oxygen atom


As you can see, oxygen has 2 of its 8 electrons in the shell closest to the nucleus, and the remaining 6 electrons - which are called valence electrons - in its second shell - this is oxygen's outermost shell. In order to complete its octet, it needs 8 electrons in its outermost shell.

Since it already has 6, it would only require 2 more electrons to complete its octet.