Near the beginning of the twentieth century, Max Planck suggested that energy changes might occur in a stepwise manner. This concept forms the basis of quantum mechanics and Einstein applied the quantum concept of energy to light and revived the idea of the particulate nature of light. This equations shows that the size of the energy steps change with the frequency or wavelength of the light. Einstein introduced the term photon to describe the smallest increments of light. In our current model of the atom, protons and neutrons are found in the nucleus and electrons are found spinning around outside the nucleus.
Electrons spin and rotate around the nucleus billions of times a second.
According to modern theory, electrons are arranged in energy levels as they rotate around the nucleus. When electrons gain or lose energy, they jump between energy levels as they are rotating around the nucleus. Since the energy of the system is restricted to certain energy values, the atom is said to be quantized. In the animated image below, it can be seen that the electrons move to a different energy state only when a specific amount of energy is added to or removed from the system. Another way of illustrating this point is with an energy diagram as presented below.