Atomic Theory

Chapter 8

The Composition of the atom

John Dalton• 1800 -Dalton proposed a modern atomic model

based on experimentation not on pure reason.

• All matter is made of atoms.• Atoms of an element are identical.• Each element has different atoms.• Atoms of different elements combine

in constant ratios to form compounds.• Atoms are rearranged in reactions.

• His ideas account for the law of conservation of mass (atoms are neither created nor destroyed) and the law of constant composition (elements combine in fixed ratios).

1803 John Dalton

• Dalton’s Model of the Atom:– Uniform Shape– Uniform Density– Indivisible

• So what did Dalton’s model look like?

• A simple design that promoted future research into atomic theory…

Discovering the Electron

• He noticed a flash of light within one of the tubes.

• Flash was produced by some form of radiation striking the light at the end of the tube.

• There were rays (radiation) traveling from the cathode to the anode in the tube.

• They called it the cathode ray b/c the ray of radiation originated from the cathode end of the tube.

• Thank you Sir William Crookes!!!!!

The Cathode Ray Tube (CRT)Developed by Sir William Crookes in

the late 1800s

Thank you Sir William Crookes!!!!!For your Accidental Discovery!!!!! 1879

• Discovery of cathode rays led to invention of the TV!!!!

•TV and computer monitor images are formed as radiation from the cathode strikes the light producing chemicals that coat the backside of the screen.

JJ Thompson 1897• Using the CRT…

Fluorescent ScreenElectromagnets (turned off)

Cathode (--)

Anode (+)

Beam of “light”

JJ Thompson 1897• When the magnets were turned on, the

beam was pulled towards the + plate

• What could Thompson conclude?+ side

-- side

• What did his experiment discover?• ELECTRONS! (negatively-charged particles)

• How did the model of the atom change?

JJ Thompson 1897

AKA The Chocolate-Chip Cookie Model

Robert Millikan 1909

• Determined the charge of an electron.

• Mass of an electron = 9.1 x 10-28 g

• Mass of an electron is extremely small.

Millikan’s Experiment

• Measured the time it took for an electrically charged oil drop to rise a measured distance when subjected to an electric field

• Measured the time it took electrically charged droplets to fall with no electric field

Oil Drop Experiment

Millikan’s Outcomes

• Determined the charge on a single electron

• Using the charge from Millikan and the charge-to-mass ratio from Thomson, many scientists determined the mass of an electron.

Discoveries from the CRT

• Wilhelm Roentgen (1895)

?

●Discovered x-rays●Killed his wife

Henri Becquerel 1896

• Roentgen’s Discovery of X-rays provoked further research by many others, including Becquerel

• Since fluorescent materials could be charged by X-rays, Henri wondered if naturally fluorescent minerals could give off X-rays….

• Like any good scientist, he set up a test…

Henri Becquerel 1896

Photographic film sealed away

Henri Becquerel 1896The photographic

paper was still exposed!

The mineral contained Uranium!

Serendipity- accidental discovery

Becquerel’s Outcomes

• Concluded something in the sample itself must be capable of “fogging” the plates; the sample must give off some kind of ray without the influence of the sun

• Rays were the products of radioactivity• 1896 - credited with the discovery of

radioactivity

● Left a sample of uranium ore in a drawer with photo plates and there was fogging on the plates

Marie and Pierre Curie

• French Scientists

• Intrigued with Becquerel’s work

• Studied radioactivity

• Marie discovered the element POLONIUM

Ernst Rutherford 1898

• After the discoveries of Becquerel and Thompson, Rutherford was dissatisfied with the atomic models and set out to disprove it.

• Using Uranium, he and his lab assistants (Geiger and Marsden) set out…

Ernest Rutherford 1911

• Studied how positively charged alpha particles interacted with solid matter.

Conducted an experiment to see if alpha particles would be deflected as they passed through a thin foil of gold. Also known as Rutherford’s Gold Foil Experiment.

Ernest Rutherford’s Gold Foil Experiment1. A narrow beam of alpha particles was

aimed at a thin sheet of gold foil.

2. A zinc sulfide coated screen surrounding the gold foil produced a flash of light whenever it was struck by an alpha particle.

1. Most all of the alpha particles passed straight through the gold foil, without deflection.

2. Some alpha particles were scattered at small angles.

A few times they were deflected at very large angles.

Rutherford’s Gold Foil Conclusions

1. Rutherford said plum pudding model was incorrect.

2. An atom consisted mostly of empty space through which the electrons move.

3. There was a tiny, dense region, called the nucleus, centrally located within the atom that contained all of an atom’s positive charge and almost all of its’ mass.

Planetary Motion Model

• Electrons orbit around the nucleus like planets orbit around the sun

• The nucleus held only protons (now)

Hans Geiger

• Student of Rutherford

• Spent long hours counting the particles that hit the screen

• Created the first electric particle counter - The GEIGER counter

• Determined that alpha particles had 2 p+ and 2n0 (1908)

Irene Curie-Joliot & Frederic Joliot

• Bombarded Be with alpha particles - formed a beam of energy with great power which was able to penetrate metals

• Thought it was new form of energy - thought it was gamma rays

James Chadwick• Found new beam of energy was unaffected

by magnetic fields - so it was neutral• Particles moved at 1/10 the speed of light -

so it was NOT radiant energy - but must be particles

• The new particles had approximately the same mass as a proton - but they had no charge

• Credited with the discovery of the NEUTRON

After Chadwick

• After neutrons were discovered - isotopes of various elements were discovered

• We will come back and pick up our story in a little bit.