The Origins The Origins of the of the

Atomic TheoryAtomic TheoryPisgah High SchoolPisgah High School

M . JonesM . Jones Rev. 1/30/021/5/04

10/14/04

The Development of the Atomic Theory

• Democritus and Dalton: atomic theory

• Crookes, Roentgen, Becquerel, Curies: early evidence for subatomic particles

• Thomson: CRT’s and the electron

• Millikan: “Oil Drop Experiment”

• Rutherford: “Gold Foil Experiment”

• Chadwick: Neutron

Democritus• Greek philosopher ~ 300 BC• Limit to “smallness”• All matter consists of tiny,

indestructible particles called atoms

• Atomos – indestructible• Aristotle and infinity

John DaltonFirst serious atomic theory

• English scientist

• Studied the properties of gases

• “Reinvented” the idea of atoms

• Published in 1803

1. Elements are composed of tiny, discrete, particles

called atoms.

Dalton’s atomic theory - 1803

2. Atoms are indivisible and indestructible and do not change their identity

during reactions.

Dalton’s atomic theory - 1803

3. Atoms of the same element are identical in mass and chemical and

physical properties. Atoms of different elements are

different.

Dalton’s atomic theory - 1803

4. Atoms combine to form compounds in simple, whole-number ratios.

Dalton’s atomic theory - 1803

Law of Definite Proportions

5. Atoms combine in different ratios to make

two or more compounds.

Dalton’s atomic theory - 1803

Law of Multiple Proportions

Dalton’s Atomic Theory1. Atoms are tiny, discrete particles 2. Atoms are indestructible3. Atoms of the same element have the

same mass and properties4. Atoms combine in simple whole-

number ratios5. Atoms in different ratios produce

different compounds.

Dalton’s Atomic Theory1. Atoms are tiny, discrete particles 2. Atoms are indestructible3. Atoms of the same element have the

same mass and properties4. Atoms combine in simple whole-

number ratios5. Atoms in different ratios produce

different compounds.

Evidence for subatomic particles

During the 19th century many discoveries were made that were later shown to involve subatomic particles.

Cathode rays, canal rays, X-rays, and then alpha, beta and gamma rays were discovered and studied.

Evidence for subatomic particles

Each helped advance the foundation of what would later become the atomic theory.

People like Crookes, Goldstein, Roentgen, Becquerel, Pierre and Marie Currie, Thomson, Millikan, Rutherford

and others all paved the way for us to be able to talk about atoms.

William CrookesStudied spectroscopy and discovered thallium.

Used vacuums to measure the mass.

Invented the radiometer.Made better vacuums.

Techniques which were used by Edison to make light bulbs.

William Crookes

Developed Developed what was what was called the called the

Crookes’ TubeCrookes’ Tube

… … which is what we which is what we now call a cathode ray tube.now call a cathode ray tube.

William CrookesUsed the cathode ray tube to to study electric fields in vacuum and discovered rays, …

which were called “cathode rays” by Goldstein, since they came from the cathode, or negative electrode.

Cathode Ray Tube

High voltageHigh voltage

Cathode Anode

Direction of cathode rays

Near-vacuum inside the glass tube

+

William Crookes

He found that the cathode rays could be deflected by a magnet.

This suggested that the cathode rays might be a

stream of charged particles.

Cathode Ray Tube

High voltage

Cathode Anode

Direction of cathode rays

+

Cathode Ray Tube

High voltageHigh voltage

Cathode Anode

Direction of cathode rays

+

Magnet

Wilhelm Roentgen

Used cathode rays to study the luminescence the rays created in certain chemicals.

To observe the faint glow, he surrounded the cathode ray tube with black cardboard.

Wilhelm Roentgen

Discovered that some barium platinocyanide was glowing even though none of the cathode rays could reach it because they were blocked by the cardboard.

Wilhelm RoentgenAn invisible radiation was coming from the cathode ray tube and passing through the cardboard.

Now we know X-rays as high energy electromagnetic radiation caused by the sudden stopping of electrons.

He called them X-rays.

Henri BecquerelWanted to see if fluorescent substances produced X-rays.

Out in the sun, he put a crystal of a fluorescent chemical on photographic film which was wrapped in black paper.

The chemical was uranium sulfate.

Henri Becquerel

Radiation penetrated the black paper.

Must be X-rays.

Because, when developed, the film was fogged.

Henri BecquerelAfter several cloudy days with the uranium sulfate and wrapped film safely in a drawer,he processed the film to see if there was any residual fluorescence.

Henri Becquerel

This was what he found.

The fogged area was even larger.

The fogging did not involve either sunlight or fluorescence.

Henri BecquerelHe studied the radiation from the uranium compound.

Found it similar to X-rays.

Could penetrate

materials and ionize air.

Henri Becquerel

Marie Curie named it

radioactivity.

The radiation was not X-rays.

It was a new kind of radiation, from a new source.But it behaved

like X-rays.

Henri BecquerelHe also found that radioactivity could be deflected by a magnet.

Could be steams of tiny charged particles.

In 1900 he decided they were electrons.

Three kinds of radioactivity

These were named by Ernest Rutherford.

• Alpha particles

• Beta particles • Gamma rays

Three kinds of radioactivity

- helium nuclei

- electrons

- high energy electromagnetic energy

• Alpha particles

• Beta particles • Gamma rays

Radioactivity …… the natural decay of unstable atoms.

… can be detected by photographic film or a Geiger counter.

… is “ionizing radiation”. Causes cells damage and mutations – cancer.

… is protected against by shielding and distance.

Properties of Radiation

Alpha, Beta and Gamma

Look at shielding, speed, hazards and

mass.

Properties of Radiation

Alpha, Beta and Gamma

Identity

Relative Mass

Relative Speed

Hazards

Shielding

Helium nucleus (2 p + 2 n).

Relatively massive and slow.

Very dangerous when inside the body. 100% absorbed.

Blocked by 2.5 cm of air, 3-4 sheets of paper or by skin.

Alpha Particles

Electrons that come from the decay of neutrons in the nucleus

Much less massive.

Much faster than alphas.

Dangerous to cells.

Blocked by metals or plastic.

Beta Particles

Electromagnetic energy, not particles.

Like light but invisible, much higher energy and shorter wavelengths.

Travel at the speed of light.Have no mass.

Gamma Rays

Can easily pass through your body, and can damage cells.

Gamma Rays

Greater penetrating power. Blocked by many inches of lead or many feet of concrete.

Alpha, Beta, Gamma

Radioactive Source

- - - - - - - - -

+ + + + + + + +

Electrically charged plates

Alpha, Beta, Gamma

Radioactive Source

Paper Lead

Aluminum foilor wood

Alpha, Beta, Gamma

Radioactive Source

Paper

Aluminum foilor wood

Lead

Alpha, Beta, Gamma

Radioactive Source

Paper Lead

Aluminum foilor wood

Cathode Ray Tube

High voltage

Cathode Anode

+

It was also used by J. J. Thomson

J. J. Thomson

• Cathode rays - cathode ray tube

• Attracted to positive electrode

• Thought they might be atoms

• Had same charge to mass ratio regardless of metal in the cathode

• Particle must be common to all matter, a subatomic particle

That particle was called the …

The electron

The electronThe electronThe electron

The electron

The ElectronDiscovered in 1897By J. J. Thompson

The term “electron” actually comes from George Stoney’s term for the “minimum electrical charge”.

After the discovery of the electron, it was assumed that this particle was the carrier of the minimum electrical charge and so the particle was called an “electron”.

J. J. ThomsonEven though Crookes and others observed and characterized cathode rays, Thomson is credited with the discovery of the electron because he recognized that it was a fundamental particle of nature -- a sub-atomic particle.

J. J. Thomson

Measured the charge to mass ratio, and found …

… that if this “minimum charge” was equal to the charge on a

hydrogen ion, then the mass of

the electron would be 1/1837th the

mass of a hydrogen atom.

J. J. ThomsonIf that were the case, then the electron would be much smaller than the smallest atom,

… showing for the first time that matter is made up of

particles smaller than atoms.

Thomson tried to measure the fundamental charge on the electron.

Robert A. Millikan

Robert A. Millikan, an American physicist, set out to determine the charge on an electron.

From 1909 through 1910, he performed what is now called

the “Oil Drop Experiment”.

HighVoltage

Cast iron pot

Robert A. Millikan

Telescope

Atomizer

Oil Drop

Cast iron pot

Radiation stripped electrons from the oil droplets. The charged droplets fell between two electrically charged plates. By adjusting the voltage, he could change the rate of fall or rise of a single oil drop. After observing hundreds of drops, he calculated the charge on a single electron.

HighVoltage

Robert A. Millikan

Telescope

Cast iron pot

Atomizer

Oil Drop

Robert A. Millikan

Charges on drops are multiples of 1.602 x 10-19 coulombs.

Robert A. MillikanThe fundamental charge on an electron is 1.602 x 10-19 coulombs.

With J. J. Thomson’s charge to mass ratio, and Millikan’s charge on the electron, we are able to compute the mass of an electron:

9.1 x 10-28 gram

Ernest Rutherford• Authority on radioactivity.• Named alpha, beta and gamma rays.• Geiger and Marsden do a series of

alpha scattering experiments. (1909)• Most alpha particles undeflected.

Few underwent large changes – some came back toward source.

• Similar to shooting at tissue paper

The Gold Foil Experiment

Top View

Side View

The Gold Foil Experiment

Alpha particle source

Gold foil Fluorescentdetector

ZnS

All of this was in a vacuum chamber.

The Gold Foil ExperimentMost of the

particles went…

…straight through the gold foil, undeflected.

The gold is mostly “empty space.”

Alpha Particles

Alpha particles are helium nuclei.

++

Two protons and

two neutrons.

The alpha particle is positively charged.

Gold Foil Experiment: Results

+

source

Small, dense, positively charged

nucleus of gold

Gold Foil Experiment: Review

+

source

The positive particles are

repelled by the nucleus.

Rutherford’s Nuclear AtomAlpha particles were repelled by…

… a small, dense, positively charged nucleus.

Almost all the mass of an atom is in the nucleus.

Electrons are located outside the nucleus. Published results in 1911.

Rutherford and the Proton1917 – 1924: Rutherford experimented with radioactivity and protons.

Bombarded the lighter elements with alpha particles. Some protons were

knocked loose - transmutation occurred.

The first person to cause a change from one element to another.

N + O + H

Rutherford and the Proton

N + O + H

7 protons

2 protons

1 proton

8 protons

9 protons 9 protons

Chadwick and the Neutron

Worked with Rutherford on alpha bombardment from 1919.

Then later on the search for a neutral particle in the nucleus.

Both disagreed with the current theory of extra protons and electrons in the nucleus.

Chadwick and the Neutron

Particles can be detected by their ability to ionize air, but neutral

particles did not ionize air.

He repeated experiments (1932) which showed an undetected radiation knocking protons out of paraffin.

The radiation consisted of neutrons.

Many more scientists contributed to the development and refinement of the

atomic theory.