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Chapter 3.2

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Objectives 1. Summarize the observed properties of cathode rays that led to the discovery of the electron. 2. Summarize the experiment carried out by Rutherford and his co-workers that led to the discovery of the nucleus. 3. List the properties of protons, neutrons, and electrons. 4. Define atom.

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Structure of the Atom Define atom: smallest particle of an element that retains the chemical properties of that element. Nucleus very small region located at the center of an atom Contains 1.Protons positive charged particle 2.Neutron particle with NO charge Area surrounding nucleus Electron cloud 1.Electrons negative charged particle Subatomic particles Visual Concept Click Here

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Properties of subatomic particles

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Discovery of the Electron Late 1800s J.J. Thomson discovered the electron through experiments with cathode- ray tubes. Robert A. Millikan 1909 Oil drop experiment Electron is 1/1837 the mass of hydrogen atom Plum Pudding Model of atom Visual Concept 2 Click Here Visual Concept 1 Click Here

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Discovery of the Atomic Nucleus In 1911 Ernest Rutherford Gold Foil Experiment Showed that there was a small dense positively charged nucleus of an atom. Visual Concept Click Here

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Composition of the Atomic Nucleus Nuclear Forces Short range proton-neutron, proton-proton, and neutron-neutron forces that hold the nuclear particles together Sizes of Atoms Nucleus compared to rest of atom - approximately size of marble compared to a large football stadium. Atom is mostly empty space All atomic nuclei are made of protons and neutrons. Atoms are electrically neutral - contain equal numbers of protons and electrons