An atom is the smallest particle of an element that can take part in a chemical reaction. Its structure has a nucleus and energy levels. The nucleus of an atom contain protons and neutrons whereas the energy levels contain electrons. For a particle containing 56 protons (atomic number) and 82 neutrons has 56 electrons since the number of protons and neutrons in an uncharged atom are equal. The atom in this case is must be an isotope of Barium since barium has an atomic number of 56 .
When the electron changes levels, it decreases energy and the atom emits photons. The photon is emitted with the electron moving from a higher energy level to a lower energy level. The energy of the photon is the exact energy that is lost by the electron moving to its lower energy level.
