Notes

John Dalton (1799-1844) developed an atomic theory in the 1800s; he was the first scientist to create atomic symbols paved the way for many others to expand and refine his theories. His most important conclusions are summarised below:
All matter is made of atoms.
Atoms cannot be broken down into anything simpler.
All the atoms of a particular element are identical to each other and different from the atoms of other elements.
Atoms are rearranged in a chemical reaction.
Compounds are formed when two or more different kinds of atoms join together.
We still teach and use many of his early theorems today.
JJ Thomson's discovery of the electron
JJ Thompson discovered the electron in 1897. This showed that the atom contained smaller pieces, whereas Dalton had thought that atoms could not be broken down into anything simpler.
Rutherford's nuclear atom
In 1911 Ernest Rutherford used experimental evidence to show that an atom must contain a central nucleus. This was further evidence that an atom contained smaller pieces.
Bohr's electron orbits
Niels Bohr further developed Rutherford's nuclear atom model. He used experimental evidence to support the idea that electrons occupy particular orbits or shells around the nucleus of an atom.

The development of the theory of atomic structure is an example of how a theory may change as new evidence is found.

JOHN DALTON
Understanding of atoms didn’t progress much beyond Demokritos’ theory until the English chemist John Dalton (1766 - 1844) started to look at it in the 1800s. Dalton did experiments, worked out some atomic weights, and invented symbols for atoms and molecules.
Dalton's theories about atoms took a long time to be accepted by scientists. Some of his ideas about gases were incorrect, and it was difficult for many years to do the experiments needed to support his theories, because atoms are too small to see.
JJ THOMPSON
In 1897 the British physicist Joseph John (J. J.) Thomson (1856–1940) discovered the electron, in his experiment Thompson interpreted the deflection of the rays by electrically charged plates as evidence of “bodies much smaller than atoms”. This showed that atoms were not the smallest things and introduced th idea of the electron, inside the atom.
ERNST RUTHERFORD
Rutherford overturned Thomson's model in 1911 with his well-known gold foil experiment in which he demonstrated that the atom has a tiny, heavy nucleus.
These deflections in his experiment were not consistent with Thomson's model. Rutherford was forced to discard the Plum Pudding model (Thomson's design) and reasoned that the only way the alpha particles could be deflected backwards was if most of the mass in an atom was concentrated in a nucleus. He then developed the 'planetary model' of the atom which put all the protons in the nucleus and the electrons orbited around the nucleus like planets around the sun.
NIELS BOHR
Niels Bohr was a Danish physicist and he got Nobel Prize in 1922 in physics for atomic structure and quantum mechanics. He was one of the most influential scientists in 20th century: these are the models main principals:

Electrons assume only certain orbits around the nucleus. These orbits are stable and called "stationary" orbits. Each orbit has an energy associated with it. For example the orbit closest to the nucleus has an energy E1, the next closest E2 and so on.
Light is emitted when an electron jumps from a higher orbit to a lower orbit and absorbed when it jumps from a lower to higher orbit.
The energy and frequency of light emitted or absorbed is given by the difference between the two orbit energies.