In 1913, Bohr proposed his Bohr model of the atom, which was a significant step forward in understanding atomic structure. He suggested that:

* Electrons orbit the nucleus in specific, quantized energy levels.

* These orbits are circular and have fixed radii, meaning electrons can only exist at certain distances from the nucleus.

* Electrons can jump between these orbits by absorbing or emitting photons of specific energies.

While Bohr's model was a valuable contribution and successfully explained some atomic phenomena, it was later replaced by the more accurate quantum mechanical model of the atom. The quantum model describes electrons as wave-like particles that exist in probability clouds rather than fixed orbits.