College Physics 10th Edition by Raymond Serway,Chris Vuille

Exercise 22

It is easy to understand how two electrons (one with spin up, one with spin down) can fill the 1 s shell for a helium atom. How is it possible that eight more electrons can fit into the 2 s , 2 p level to complete the 1 s 2 s 2 2 p 6 shell for a neon atom