Chapter 4, Section 2 Sub-Atomic Particles and Nuclear Atoms

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


Self Taught Class

• Who it is?

• When?

• What did He do?

• How was it important to understanding the Atom?


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



• 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


Crookes CRT


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


Section 1 Development of the Atomic TheoryChapter 4

Thompson’s Cathode-Ray Tube Experiment


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




• 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



• 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



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


Section 1 Development of the Atomic TheoryChapter 4

Rutherford’s Gold-Foil Experiment


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


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



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



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


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


Summary of Models

Documents

Chapter 4, Section 2 Sub-Atomic Particles and Nuclear Atoms