An atom contains protons and neutrons in the nucleus surrounded by electrons.

The energy of an electron depends on its location with respect to the nucleus.

Because the energy is quantized the electrons can only occupy specific regions called shells.

The shells are numbered in order of increasing energy.

n is called the principal quantum number.

The energy levels in an atom are similar to the rungs of a ladder, but they get closer together as they get farther from the nucleus.