The model below shows an atom of an element. 10 light gray and 8 dark gray balls sit at the center with 2 concentric black rings around them. The inner ring has 2 medium gray balls on it, and the outer ring has 6 medium gray balls on it. At bottom, a dark gray ball = +1, a light gray ball = 0, a medium gray ball = negative 1.

What is the atomic number of this atom?

6
8
9
16