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Billiard ball model
Based on all his observations, Dalton proposed his model of an atom, he defined an atom to be a ball-like structure, as the concepts of atomic nucleus and electrons were unknown at the time. -
Main contribution
In fact, it was this very observation that is believed to be the first time that Dalton hinted at the supposed existence of atoms. In the paper which addressed gas absorption in water.
Dalton proposed that each chemical element is composed of atoms of a single, unique type, and though they cannot be altered or destroyed by chemical means, they can combine to form more complex structures. -
What was wrong?
In Dalton's Atomic Theory the indivisibility of an atom was proved wrong: an atom can be further subdivided into protons, neutrons and electrons. However an atom is the smallest particle that takes part in chemical reactions. According to Dalton, the atoms of same element are similar in all respects. -
Description of the model.
Dalton discovered that certain gases could only be combined in certain proportions.
These experiments built upon two theories that had emerged near the end of the 18th century which dealt with chemical reactions. The first was the law of conservation of mass.
The second was the law of definite proportions, first proven by the French chemist Joseph Louis Proust in 1799.
Studying these laws and building on them, Dalton developed his law of multiple proportions. -
What was wrong?
The main problem with Rutherford's model was that he couldn't explain why negatively charged electrons remain in orbit when they should instantly fall into the positively charged nucleus. This problem would be solved by Niels Bohr in 1913. -
Nuclear atom / planetary model of the atom
The model described the atom as a tiny, dense, positively charged core called a nucleus, in which nearly all the mass is concentrated, around which the light, negative constituents, called electrons, circulate at some distance, much like planets revolving around the Sun.
Most alpha particles were observed to pass straight through the gold foil, which implied that atoms are composed of large amounts of open space. -
Main contribution
The nucleus was postulated as small and dense to account for the scattering of alpha particles from thin gold foil, as observed in a series of experiments performed by undergraduate Ernest Marsden under the direction of Rutherford and German physicist Hans Geiger in 1909.
The Rutherford model supplanted the “plum-pudding” atomic model of English physicist Sir J.J. Thomson, in which the electrons were embedded in a positively charged atom like plums in a pudding.
