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In 1897, [[JJ Thomson]], through his work on [[cathode rays]], discovered the electron and its subatomic nature (i.e., its lightness compared with the mass of atoms), which destroyed the concept of atoms as being indivisible units. Later, Thomson also discovered the existence of isotopes through his work on ionized gases.
In 1897, [[JJ Thomson]], through his work on [[cathode rays]], discovered the electron and its subatomic nature (i.e., its lightness compared with the mass of atoms), which destroyed the concept of atoms as being indivisible units. Later, Thomson also discovered the existence of isotopes through his work on ionized gases.
[[Image:Bohr_Model.svg.png]]
[[Image:Bohr_Model.svg.png|right|thumb|A Bohr model of the
hydrogen atom, showing an electron jumping between fixed orbits and emitting a [[photon]] of energy with a specific frequency.]]
Thomson believed that the electrons were distributed evenly throughout the atom, balanced by the presence of a uniform sea of positive charge. However, in 1909, the [[gold foil experiment]] was interpreted by [[Ernest Rutherford]] as suggesting that the positive charge of an atom and most of its mass was concentrated in a nucleus at the center of the atom ([[Rutherford model]]), with the electrons orbiting it like planets around a sun. In 1913, [[Niels Bohr]] added [[quantum mechanics]] into this model, which now stated that the electrons were locked or confined into clearly defined orbits, and could jump between these, but could not freely spiral inward or outward in intermediate states.
Thomson believed that the electrons were distributed evenly throughout the atom, balanced by the presence of a uniform sea of positive charge. However, in 1909, the [[gold foil experiment]] was interpreted by [[Ernest Rutherford]] as suggesting that the positive charge of an atom and most of its mass was concentrated in a nucleus at the center of the atom ([[Rutherford model]]), with the electrons orbiting it like planets around a sun. In 1913, [[Niels Bohr]] added [[quantum mechanics]] into this model, which now stated that the electrons were locked or confined into clearly defined orbits, and could jump between these, but could not freely spiral inward or outward in intermediate states.