An atom with the electron configuration 2-8-8-2 has an incomplete

3rd principal energy level

Explanation:

Principle energy that explains the energy of the electrons and positions of electrons of a particular atom. There are four energy levels fist to fourth that are made of sublevels and that is made up of s, p, d and f orbitals.

First principal energy level -  2

Second principal energy level - 8

Third principal energy level - 18

Fourth principal energy level -  32

3rd energy level has 9 orbitals, which has 18 electrons

2 in the s orbital,

6 in the three p orbitals, and

10 in the five d orbitals.

So, here there are only 8 electrons in the third principal energy level.