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Chapter 4, Section 2
•Sub-Atomic Particles and Nuclear Atoms
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Accidental Discoveries?• Does anything get discovered by
accident?
• Yes– Vulcanized rubber– Aspartame (Nutrasweet)– Electrons
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Self Taught Class
• Who it is?
• When?
• What did He do?
• How was it important to understanding the Atom?
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Discovering the Electron
• Sir William Crookes, early 1800’s– What is the relationship between electricity
and matter?• Static from combs• Static from carpets
• Recent inventions:– Vacuum pump– Cathode Ray Tube (CRT)
• Cathode (+) at one end of vacuum tube• Anode (-) at other end
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Discovering the Electron
• Crookes was in a darkened room.– Noticed flashes of light within his tube (coated
inside with light producing chemicals)– Further work: “rays” going from cathode end
to anode end (hence cathode ray tube)– Cathode Ray Tube is basis for TV and
computer monitors
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Crookes CRT
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Section 1 Development of the Atomic Theory
Thompson’s Discovery of Electrons • Thompson experimented with a cathode-ray tube. • He discovered negatively charged particles known as
electrons.
Chapter 4
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Section 1 Development of the Atomic TheoryChapter 4
Thompson’s Cathode-Ray Tube Experiment
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Thompson’s Model
• Thompson proposed a new model of the atom.– electrons are mixed
throughout an atom, like plums in a pudding (or raisins in raisin bread).
– Called Plum Pudding model
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Discovering the Electron
• By late 1800’s Further work led to conclusion that:– Cathode Rays were actually stream of
charged particles– Particles carried a negative charge– These particles were found in all matter– Particles were called ‘electrons’
• CRISIS: Dalton was wrong, Atoms did have smaller particles
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Discovering the Electron
• 1909 – Robert Millikin (US)– Determined charge of an electron– Determined mass of an electron
• 9.11 X 10-28g = 1/1840 mass of a hydrogen atom
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Section 1 Development of the Atomic Theory
Rutherford’s Atomic “Shooting Gallery”
• In 1909, Ernest Rutherford aimed a beam of small, positively charged particles at a thin sheet of gold foil. The next slide shows his experiment.
• Surprising Results Rutherford expected the particles to pass right through the gold in a straight line. To Rutherford’s great surprise, some of the particles were deflected.
Chapter 4
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Section 1 Development of the Atomic TheoryChapter 4
Rutherford’s Gold-Foil Experiment
Copyright © by Holt, Rinehart and Winston. All rights reserved.
The Nuclear Atom
• Rutherford concluded Thompson was wrong:– There must be a tiny, very dense region of the
atom, called the ‘nucleus’• Must be very dense (like all the mass of an atom)• Must have a positive charge to keep the electrons
attracted
– Between atoms and nucleus must be a lot of empty space
• How Much?– Nucleus the size of a quarter has electrons over 1 mile away
Copyright © by Holt, Rinehart and Winston. All rights reserved.
The Nuclear Atom
• Rutherford Model Explains:– Why alpha particles (electrons) bend on their
way through nucleus– Why some alpha particles are deflected at
very sharp angles
• Did not explain all of the Atom’s Mass
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Section 1 Development of the Atomic Theory
Where Are the Electrons?
• Far from the Nucleus Rutherford proposed that in the center of the atom is a tiny, positively charged part called the nucleus.
Chapter 4
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Discovering Protons and Neutrons
• 1919 Rutherford Later Experiments– Concluded nucleus must contain positive
particles called ‘protons’– With co-worker James Chadwick showed
nucleus also contained a neutral particle called ‘neutron’
• Mass of neutron almost same as proton• No electrical charge
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Summary to Date
• Atoms are composed of:– Protons (+ charge, 1 mass unit)– Neutrons (no charge, 1 mass unit)– Electrons (- charge, very little mass)
• Most of an atom’s size is electrons moving through empty space– Electrons are held to nucleus by +/- electrical
attraction
Copyright © by Holt, Rinehart and Winston. All rights reserved.
Summary of Models