What did Ernest Rutherford do for atomic theory?

Answer 1

Rutherford advanced atomic theory by suggesting a "next step" in atomic structure. We now know that atoms have a nucleus with neutrons and protons in it, and that's something over 99.99% of the total mass of the atom. The electrons form up in shells in the electron cloud. But what about back then?

The J.J. Thompson model of the atom was one called "plum pudding" with the massive elements inside just "scattered around" like plums in a pudding. This Thompson model had no central mass. Rutherford, based on his "thinking through" the results of experimentation, figured out that the mass of the atom must be concentrated in a central location with electrons orbiting around it. His contribution stands as a monument to his efforts and as an important "stepping stone" to further refinement of the atomic model.

Remember that scientists see themselves standing on the shoulders of earlier investigators, and as Rutherford stood on Thompson's shoulders, so did Bohr stand on Rutherford's. Thus is science advanced. A link can be found below.

Physicists knew that the atom contained positive and negative charges in equal numbers, but not how they were distributed. It was believed that there was a cloud of positive charge, with the negative electrons distributed all through it like currants in a plum pudding. There was no real evidence for the "plum pudding" model, it just seemed reasonable on mathematical grounds.

Rutherford tried firing a narrow stream of (positively charged) alpha particles at a thin gold foil and detecting where they came out. He thought that the particles would go through the foil, but some would be deflected slightly by passing close to an electron in an atom. Many were, but some were deflected by large amounts, even to the extent of being deflected right back towards the source of particles.

This was astounding ! Rutherford himself said that it was like firing a fifteen inch shell at a piece of tissue paper and having it bounce back. Rutherford decided that the only way this could happen was if all the positive charges were concentrated in one small place, with the negative electrons orbiting this "nucleus" like planets round a star. This is how atoms are structured. Rutherford discovered the proton; he later suggested that there might be a neutral particle called a neutron. It was one of Rutherford's students (Chadwick) who actually discovered the neutron later.

Rutherford made the atomic theory in 1919 when he was working for J.J. Thompson. He found the atomic nucleas but he also made it so that people knew what matter is made up of.

Answer 2

Albert Einstein was the first to prove the existence of atoms in his seminal paper on Brownian motion (this, incidentally, also proved the existence of molecules). He is probably the most influential figure in the history of Quantum Mechanics, beginning with his 1905 paper on the Photoelectric Effect. This seminal work revolutionized physics by postulating that light was a particle. He, more than Max Planck, introduced the concept of the quantization of energy in atomic mechanics. Einstein proposed the photon, the first force-carrying particle discovered for a fundamental interaction, and put forward the notion of wave-particle duality, based on sound statistical arguments 14 years before De Broglie's work. He was the first to recognize the intrinsic randomness in atomic processes, and introduced the notion of transition probabilities, embodied in the A and B coefficients for atomic emission and absorption. He also preceded Born in suggesting the interpretation of wave fields as probability densities for particles, photons, in the case of the electromagnetic field. Finally, stimulated by Bose, he introduced the notion of indistinguishable particles in the quantum sense and derived the condensed phase of bosons, which is one of the fundamental states of matter at low temperatures. His work on quantum statistics in turn directly stimulated Schrodinger towards his discovery of the wave equation of quantum mechanics. It was only due to his rejection of the final theory that he is not generally recognized as the most central figure in this historic achievement of human civilization.

Source: http://astro1.panet.utoledo.edu/~ljc/einstein_camop2.pdf

Source: Professor A. Douglas Lecture at John Hopkins on Einstein and Quantum Mechanics https://www.hopkins.edu/podium/default.aspx?t=52562&a=292745

Source: http://www.sciencefriday.com/blogs/10/31/2013/einstein-s-monster.html