As they got closer to the outer portion of the atom, the positive charge in the region was greater than the neighboring negative charges and the electron would be pulled back more toward the center region of the atom.
However, this model of the atom soon gave way to a new model developed by New Zealander Ernest Rutherford - about five years later. Thomson did still receive many honors during his lifetime, including being awarded the Nobel Prize in Physics in and a knighthood in In , Rutherford and coworkers Hans Geiger and Ernest Marsden initiated a series of groundbreaking experiments that would completely change the accepted model of the atom.
They bombarded very thin sheets of gold foil with fast moving alpha particles. Rutherford found that a small percentage of alpha particles were deflected at large angles, which could be explained by an atom with a very small, dense, positively-charged nucleus at its center bottom. According to the accepted atomic model, in which an atom's mass and charge are uniformly distributed throughout the atom, the scientists expected that all of the alpha particles would pass through the gold foil with only a slight deflection or none at all.
Some were even redirected back toward the source. No prior knowledge had prepared them for this discovery. In a famous quote, Rutherford exclaimed that it was "as if you had fired a inch [artillery] shell at a piece of tissue and it came back and hit you. Rutherford needed to come up with an entirely new model of the atom in order to explain his results. Because the vast majority of the alpha particles had passed through the gold, he reasoned that most of the atom was empty space.
In contrast, the particles that were highly deflected must have experienced a tremendously powerful force within the atom. He concluded that all of the positive charge and the majority of the mass of the atom must be concentrated in a very small space in the atom's interior, which he called the nucleus.
In fact, most of the matter is empty space. In , Rutherford and coworkers Hans Geiger and Ernest Marsden initiated a series of groundbreaking experiments that would completely change the accepted model of the atom. They bombarded very thin sheets of gold foil with fast moving alpha particles. Alpha particles, a type of natural radioactive particle, are positively charged particles with a mass about four times that of a hydrogen atom.
Figure 1. B According to the plum pudding model top all of the alpha particles should have passed through the gold foil with little or no deflection. Rutherford found that a small percentage of alpha particles were deflected at large angles, which could be explained by an atom with a very small, dense, positively-charged nucleus at its center bottom. Surprisingly, while most of the alpha particles were indeed undeflected, a very small percentage about 1 in particles bounced off the gold foil at very large angles.
Some were even redirected back toward the source. No prior knowledge had prepared them for this discovery. Rutherford needed to come up with an entirely new model of the atom in order to explain his results. Because the vast majority of the alpha particles had passed through the gold, he reasoned that most of the atom was empty space. In contrast, the particles that were highly deflected must have experienced a tremendously powerful force within the atom.
The nucleus is the tiny, dense, central core of the atom and is composed of protons and neutrons. The Rutherford atomic model was also known as the "Rutherford nuclear atom" and the "Rutherford Planetary Model". In , Rutherford described the atom as having a tiny, dense, and positively charged core called the nucleus. Rutherford established that the mass of the atom is concentrated in its nucleus.
The light, negatively charged, electrons circulated around this nucleus, much like planets revolving around the Sun. Rutherford's model varied from Dalton's and Thompson's models in that he considered that most of the atom was empty space, whereas in the earlier models the atom was considered solid.
The Bohr atomic model After working with both Thomson and Rutherford, Niels Bohr, in , realised that Rutherford's model was unstable. According to classical theory, electrons moving on an orbit should emit electromagnetic radiation and the electrons would therefore lose energy and spiral into the nucleus. Bohr realised this was not the case because atoms did not continually glow or spontaneously disappear.
Instead, Bohr suggested that an atom's electrons move in orbits of fixed size and energy and that the energy of an electron depends on the size of the orbit. Radiation could only be emitted from an atom when an electron dropped from a higher energy orbit to another lower orbit.
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