This modern-day theory of the structure of the atom is a result of the works of many past scientists, both chemists and physicists. These include John Dalton, Joseph J. Thomson, E. Goldstein, Ernest Rutherford, James Chadwick and Niels Bohr.
First in 1808, John Dalton came up with the Atomic Theory. He believed that atoms were the smallest particle of matter that could take place in a reaction. He also believed that they were indivisible and could not be created or destroyed.
Then in 1897, J. J. Thomson discovered the electron. He found them to be tiny, negatively charged particles. He did this by conducting experiments with Cathode Ray Tubes (CRT). This led him to believe that atoms were divisible. A year later (1898) he came up with the “plum pudding” model of the atom, which showed electrons being fixed in a positively charged sphere.
2 years later, in 1900, E. Goldstein discovered the proton. He did this by conducting Anode Ray Experiments. He believed that since atoms had an overall neutral charge, they had to contain positive particles (protons) to balance the charges of the electrons.
In the year 1911, Ernest Rutherford proposed the nuclear model of the atom. Under Rutherford’s guidance, Hans Geiger and Ernst Marsden, his understudies, conducted an experiment, which led to the development of the model.
Geiger and Marsden set up thin metal foils and then subjected these sheets of foil, to beams of helium nuclei (42He) known as alpha particles (α). They observed the experiment very closely under microscope and made observations as necessary. They observed that most of the alpha particles passed straight through the foil. They also observed that a few particles were deflected slightly, on their way through the foil. To their surprise, they noticed that a very small number of alpha particles did not pass through the foil. These particles were deflected right back in the direction the came from.
Using his knowledge of electrostatic repulsion, Rutherford was able to explain the observed behaviour of the particles and used this to create his model of the atom. He explained that if a positive alpha particle was to be deflected, they must be another positively charged body repelling it. He called this positive charged body, the nucleus. Since only a few particles experienced any type of deflection, Rutherford believed that this nucleus had to be very small. But, since it repelled some particles so strongly, it was also believed that this nucleus had some mass. As a result of the high number of particles that passed straight through the foil, it was thought most of the atom was empty space. Knowing that atoms had an overall neutral charge, Rutherford also explained that electrons existed in the empty space around the nucleus.
Rutherford’s nuclear model of the atom showed things:
- atoms have a tiny central area called the nucleus which is positively charged
- the nucleus is very dense compared to the rest of the atom and accounts for 99.97% of the mass of the atom
- negatively charged electrons exist in the empty space around the nucleus, orbiting it
Ruthersford’s Nuclear Model of the atom is shown in Figure 1 below.
In 1932, James Chadwick discovered neutrons. He proposed that atoms contained neutrally charged particles (neutrons) in their nucleus, which added to the mass of the nucleus and the atom itself.
The structure of the atom widely accepted today is based on Rutherford’s model with work by James Chadwick and Niels Bohr also contributing.
