Energy level in an atom refers to the energy due to atoms bound in an electric field. When an electron jumps from higher energy to lower energy it releases a photon, which has a wavelength.

Energy level is Energy level.

The second principle energy level is the energy level that comes after the first principle energy level. The second energy level is farther from the atomic nucleus and contains the 2s and 2p sublevels.

The outermost principle energy level for strontium is the 5th energy level.

Carbon has four electrons in the outermost energy level, which is energy level two. It needs eight electrons to have this energy level filled.