Whose idea introduced electron shells and explained periodic patterns?

Niels Bohr introduced his atomic model in 1913, placing electrons in fixed, quantized energy levels or shells. That idea helped explain why elements recur in groups with similar properties: the outermost electrons largely determine chemical behavior. Bohr’s model was not the final word in atomic physics, but it was a major step toward understanding the periodic table. It connected atomic structure to chemistry in a way earlier models had not.