What is one sentence to describe the contributions of John Dalton, J.J. Thomson, Ernest Rutherford and Niels Bohr to the atomic theory?
John Dalton proposed the first atomic theory, J.J. Thomson discovered the electron, Ernest Rutherford discovered the nucleus and Niels Bohr is known for the Bohr model in which electrons are in orbits outside the nucleus.
Here's some more detail:
In In 1803 John Dalton proposed the first atomic theory, based on the idea from the early Greek philosopher Democritus that matter is made of tiny particles. Dalton described atoms as tiny, indivisible and indestructible particles.
In 1897 J.J. Thomson, while experimenting with a cathode ray tube, identified the negatively charged electron. From this he proposed what later became known as the Plum Pudding model in which the electrons are scattered throughout a sphere of positive charge like the raisins in a plum pudding.
In 1911 Ernest Rutherford conducted the famous Gold Foil Experiment in which he directed positively charged alpha particles at a sheet of gold foil and observed their paths using a detecting screen. He found that nearly all of the alpha particles passed through with their paths unaltered, while a few were deflected to one side or straight back. From this he concluded that the positive charge, which repelled the alpha particles, was concentrated in very small areas in the centers of atoms. He revised Thomson's atomic model, producing the first nuclear model of the atom.
In 1913 Niels Bohr observed the light given off by hydrogen atoms when electrified. He observed that this light, when passed through a spectrum, produced only a few lines of specific wavelength rather than a rainbow-like continuous spectrum. His explanation was that the light was produced by electrons absorbing energy while in the ground state, then moving farther from the nucleus, then giving off the energy in the form of light when they returned to the ground state. He further explained that electrons' energy is "quantized", meaning that they can only be certain specific distances from the nucleus. From this he developed the Bohr Model of the atom, in which electrons have specific paths he called orbits.
The more recent Quantum Mechanical model still accounts for the observation that electrons have energy that is quantized, but describes regions of probability of finding an electron rather than specific orbits. These probability functions are called wave equations.