Models of Atomic Structure

Dalton’s Small Dense Sphere

The Structure of the Atom

• The atom is defined as the smallest particle of an element that retains all the chemical properties of that element.

• In the late 1800’s, scientists believed that atoms were electrical in nature.

• Sir Crookes set out to prove the electrical nature of atoms by using a cathode-ray tube.

• In a cathode-ray tube, electrons travel as a ray through a gas at low pressure from the cathode (negative electrode) to the anode (positive electrode).

• Crookes attached the cathode and the anode to a power source. When he turned the power on, a glowing beam appeared between the electrodes.

• He tested the glowing beam for magnetic effect by using a magnet. When the magnet was placed near the tube, the cathode ray was deflected the same way a wire carrying electric current would. Electric current was known to have a negative charge, so Crookes determined that the cathode ray was also negative .

• Crookes also noticed a shadow behind the anode, which meant that the source of the ray was the cathode (the ray was coming from the negative electrode).

• He determined with certainty, that the ray must have a negative charge.

Joseph John Thomson

• In 1897, J. J. Thomson performed a similar experiment. He used a cathode-ray tube with an anode and a cathode, but at one end he put a florescent screen. When the cathode ray hit the screen, the ray left a mark.

• When he used a heavier gas in the tube, the ray left an even bigger mark. He concluded the larger the mark on the screen, the larger the negative charge.

• Thomson later discovered that beside the cathode ray was another ray. The second ray was positive. Thomson discovered positively charged particles.

• Thomson’s improved model of the atom had negative and positive particle. This atom was commonly referred to as the plum pudding model.

Robert Millikan

• In 1909, Robert Millikan determined the mass of an electron to be 9.109 x 10-31 kg. His experiment also confirmed that the electron carried a negative charge.

• His experiment was called the oil drop experiment.

Becquerel• In 1896, Becquerel was experimenting with

black photography paper and uranium. He would spread the uranium on the paper and expose the paper/uranium to sunlight.

• The paper would be overexposed as expected, but under the uranium was not exposed.

• More importantly, around the spots where the uranium was, there were rays.

• Becquerel discovered radioactivity.

Ernest Rutherford

• In 1911, Rutherford took the research on radioactivity even further. He found alpha particles (high speed positively charged particles). He also found beta particles (high speed negatively charged particles).

• Rutherford’s experiment is called the gold foil experiment.

• In the gold foil experiment, Rutherford shot alpha particles at a thin piece of gold foil. Surrounding the gold foil was a zinc sulfide screen.

• Each time an alpha particle hit the gold foil, a flash of light was produced. Some of the particles passed through the metal and hit the screen as expected. Some of the particles deflected a bit, which was also expected.

• What caught Rutherford’s eye was the particles that came back and hit the screen in front of the foil.

• Rutherford rethought the model of the atom. He put a cluster of positive particles in the center of the atom (nucleus) and surrounded it with negative particles.

• When the emission hit the negative particles, it went straight through to the screen. When the emission came near the nucleus, it was deflected a bit. It was when the emission hit the nucleus that it bounced right back.

• In 1932, Irene Joliet Curie and her husband, Frederic, bombarded Beryllium with alpha particles. A high powered beam was formed.

• Later in 1932, Sir James Chadwick tested Mme. Curie’s high powered penetrating beam. He put a magnet around the beam and found that it did not react. Chadwick discovered a neutral particle in the atom.

• The neutral particle is the neutron and it has relatively the same mass as the proton, but does not have a charge.