1897 the discovery of the electron proved there were individual particles that make up the atom.
In 1900, the German Physical Society received a presentation by Max Plank on his version of the theory where he made the conjecture that energy was made of individual units which he referred to as quanta. Plank took his version of the quantum theory a step further and derived a universal constant which famously became known as Planck’s constant which is used to describe the sizes of quanta in quantum mechanics. Planck’s constant states that the energy of each quantum is equal to the frequency of the radiation multiplied by the universal constant (6.626068 × 10-34 m2 kg / s).
In 1905, Albert Einstein theorized that not just the energy but the radiation was also quantized in the very same manner and summarized that an electromagnetic wave such as light could be described by a particle called the photo with a discrete energy dependent on it’s frequency.
Ernest Rutherford discovered that most of the mass of an atom resides in the nucleus in 1911. Niels Bohr refined the Rutherford model by introducing different orbits in which electrons spin around the nucleus.
In 1924, the development of the principle of wave-particle duality by Louis de Broglie stated that elementary particles of both matter and energy behave, depending on the conditions, like particles or waves.