He realized that Rutherford's model ... hydrogen atoms. Bohr's theory that electrons existed in set orbits around the nucleus was the key to the periodic repetition of properties of the elements.

Quantum physics began in 1900 when the German physicist Max Planck determined the scale of quantum phenomena. In 1905, Albert ...

Key fact Isotopes are forms of an element that have the same number of protons but different numbers of neutrons. There are three isotopes of hydrogen: hydrogen, deuterium (hydrogen-2) and tritium ...