1. The structure of the atom
Chapter 3.2
The structure of the atom

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

3. 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
Protons – positive charged particle
Neutron – particle with NO charge
Area surrounding nucleus
Electron cloud
Electrons – negative charged particle
Subatomic particles
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4. Properties of subatomic particles

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

6. 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

7. Composition of the Atomic Nucleus
All atomic nuclei are made of protons and neutrons.
Atoms are electrically neutral - contain equal numbers of protons and electrons
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