Chapter 4: Discovery of Chapter 4: Discovery of Atomic StructureAtomic Structure

Prentice Hall © 2003 Chapter 2

The Discovery of Atomic The Discovery of Atomic StructureStructure

• An ancient Greek named Democritus was the first to say that matter consists of indivisible parts.

• Named parts of matter “atomos.”• Later scientists realized that the atom

consisted of charged particles.

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– Each element is composed of indivisible atoms.– All atoms of an element are identical.– In chemical reactions, atoms are rearranged, not

changed.– Compounds are formed when atoms of more than

one element combine.– Which of these hold true today?

John Dalton’s Atomic John Dalton’s Atomic Theory of MatterTheory of Matter

Cathode Ray Tube Cathode Ray Tube ExperimentExperiment

• 1897: J.J. Thomson performs cathode ray tube experiment.

• Discovers electron.• Plum pudding model of atom.

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The Discovery of Atomic The Discovery of Atomic StructureStructure

Cathode Rays and Electrons

Thomson’s Plum Pudding Thomson’s Plum Pudding ModelModel

• The atom consists of neutral, positively, and negatively charged particles.

• Thomson assumed all these particles were found in a sphere.

The Discovery of Atomic The Discovery of Atomic Structure: RadioactivityStructure: Radioactivity

• 3 types of radiation: • Alpha particles have 2+ charge• Beta particles have 1- charge (electrons)• Gamma rays have no particles, no

charge

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The Discovery of Atomic The Discovery of Atomic StructureStructure

Radioactivity

Gold Foil ExperimentGold Foil Experiment

• Ernest Rutherford carried out gold foil experiment.

• Alpha particles were shot through a piece of gold foil.

• Most of the lpha particles went straight through the foil without deflection.

• Some lpha particles were deflected at high angles.

Gold Foil ExperimentGold Foil Experiment

• Most alpha particles go through foil undeflected.

• Most of the atom must be empty space.• Center of atom must be small, dense, and

positively charged. • Discovery of nucleus!

• Rutherford changes Thomson’s Plum Pudding model as follows:– atom is spherical, has

positive charge at center, electrons spread around center.

Rutherford’s Nuclear Rutherford’s Nuclear AtomAtom

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The Modern View of The Modern View of Atomic StructureAtomic Structure

• The atom has protons, electrons, and neutrons.• Protons have 1+ charge, are in nucleus.• Electrons have 1- charge, are outside nucleus.• Neutrons have no charge, are in nucleus.• Protons and neutrons have almost same mass.• Electron mass MUCH smaller.• Nucleus is small and dense. • Most of the mass of the atom is due to the nucleus.