1. Atomic Structure Objectives: 1. Discuss how atomic structure is related to electricity. 2. Explain what studies of cathode rays and radioactivity revealed about atoms. 3. Discuss Rutherford's alpha-scattering experiment and how it showed the existence of the nucleus. Key Terms: cathode ray, cathode ray tube, electron, radioactivity, nucleus

2. Introduction As stated in a previous chapter, there are around 100 naturally occurring elements. The average size of an atom is 8x10 -11 m. At this size you could place 100,000,000 atoms side-by-side to form a line 1cm long. This fact makes the discoveries of the following individuals even more amazing!

3. Proving the structure of the atom The cathode ray tube (CRT) was developed in 1897 by J.J. Thompson. In a CRT a stream of electrons are generated and flow from the cathode to the anode. Thompson used this device to test the charge of the particles.

4. Thompson found that if he brought a positively charged magnetic pole toward the side of the tube the beam attracted toward the magnet and if he brought the negatively charged side of the magnet toward the ray it deflected. From this experiment Thompson concluded that: the movable particles of the atom have a negative charge The name electron was given to the tiny charged particle ◦ The size & charge of the electron was further quantified by Robert Millikan in 1916.  size = 1/1840 of a hydrogen atom  charge = 1 unit of negative charge

5. Henri Becquerel - Radioactivity Radioactivity was accidentally discovered by Henri Becquerel in 1896 when he placed a piece of uranium on a piece of undeveloped photographic film. called the property radioactivity ◦ the spontaneous emission of particles from a substance

6. Types of Atomic Radiation In the early 1900s Ernest Rutherford conducted experiments on radioactive elements much like the CRT experiments and found the following three types of radiation: alpha - deflected toward the negative plate beta - deflected toward the positive plate gamma - not affected by the presence of charged plates These experiments proved the presence of positively charged particles.

7. Rutherford’s Experiment Rutherford experimented further by concentrating an alpha beam at a piece of thin gold foil. Rutherford noticed that most of the rays passed straight through the foil as if it wasn't there. Some (~1 in 8000) deflected in various directions. most of an atoms mass is concentrated in a positively charged core ◦ Rutherford’s conclusion  The center of the atom is composed of protons & neutrons - called it the nucleus  the nucleus is only a tiny fraction of the size of the atom  most of the size of the atom consists of space where the electrons move around the nucleus