What causes the elements to emit light energy?
Simple answer: Valence electrons losing energy.
Atoms, the most basic unit of matter, contain charged particles call electrons and protons, along with neutral particles called neutrons. Protons and neutrons are found in the nucleus of an atom.
Electrons are arranged in specific areas called shells (or orbits). When the electrons are excited (for example, by heat), they jump from one shell to another. Once they fall back they start to emit photons--light. Each element has a different color based on how many valence electrons it has.