- Thompson discovered electrons using cathode ray tubes. Rutherford discovered the nucleus and that it is positively charged through gold foil experiments. - Chadwick discovered neutrons by observing particles in a beam that were not deflected by a magnetic field. - Atoms are made up of protons, neutrons, and electrons. Protons and neutrons reside in the nucleus, while electrons orbit the nucleus. The number of protons defines the element.

1. Structure of Atoms 3.2

2. Who discovered what? Thompson discovered electrons by using cathode ray tubes. Rutherford discovered the nucleus . He also reasoned that it was positively charged. Gold foil experiments. Chadwick discovered that a beam was not deflected in a magnetic field. The particles in the beam were named neutrons . Read pages 79-82

3. Subatomic Particles Proton + 1.673 x 10 -27 kg. Neutron : no charge. 1.675 x 10 -27 kg. Electron – 9.109 x 10 -31 kg

4. The AMU AMU stands for Atomic Mass Unit. Because the subatomic particles are so small the AMU was created to describe them. Proton = 1 AMU Neutron = 1 AMU Electron = 0 AMU

5. Atomic # & Atomic Mass The atomic # of an element is the same as the number of protons in one atom of that element. The atomic mass of an element is the number of protons + the number of neutrons in one atom of that element.

6. The Periodic Table Symbols on the periodic table hold a lot of information. Protons Electrons Mass Element symbol Neutrons? O 8 15.9994 2s 2 2p 4

7. C 6 12.011 S 16 32.066 Zn 30 65.39 H 1 1.007 Si 14 28.085 Li 3 6.941

8. Atomic Structure and Symbols It is common notation to write an element’s atomic number and mass number next to their symbol. Br 80 35

9. Isotopes Some atoms of the same element have different numbers of neutrons. They are called isotopes. Example: Both helium because they both have 2 protons. He He 4 3 2 2

10. Isotopes Elements and isotopes may also be written with the mass number followed by the name of the element. Examples: Oxygen-16 = normal oxygen, 8 protons, 8 neutrons. Oxygen-17= isotope of oxygen, 8 protons, 9 neutrons