The existence of an atom was first suggested by Democritus, the idea is then developed in 1803 by John Dalton.
According to Dalton's atomic theory, matter is made up of atoms which are inseparable and can not be destroyed further. For a given element, all atoms have identical mass and properties. Atoms of two different elements combine to form compounds and rearrangement of these atoms results in chemical reaction.
Energy levels inside an atom are the specific energies that electrons can have when energy occupies specific orbitals. Electrons can be excited to higher energy levels by absorbing energy from the surroundings, an equivalent light is emitted when an electron returns from a high energy state to a lower one. Representation of this diagrammatic is known as the energy level diagram.
This reaction is decomposition. It is the breakdown of a compound into simpler and smaller elements.
