Democritus of Abdera

Ancient Greece - 4th century BC

first suggested the existence of tiny fundamental particles that make up matter.

atoms = indestructible

did not agree with the “current” sci theory - but did not explain chemical behavior - and lacked experimental support.

John Dalton

1766-1844

Eaglesfield, England

Father of modern atomic theory

Pro: advance in the description and nature of chemical reactions.

Con: did not hold up when subatomic particles were disovered

Dalton’s Atomic Theory

1. All elements are composed of tiny individual particles - called atoms.

2.All matter is composed of combinations of these atoms.

3.Atoms of different elements are different.

4.Atoms of the same element have the same size, mass, and form.

Dalton’s Atom

The atom is a solid indestructible sphere.

William Crookes

1832-1919

British physicist

investigated why gases glow

Crookes’ Tube

Crookes’ Experiment

used a partially evacuated tube with a low pressure gas and ran a current through it

power on: a green glow was emitted as a beam of light ran across the tube

If the charge on the plates as switched . . .

Crookes’ OutcomesIt was a beam of particles?

How did he know?

Determined the particles had a negative charge - due to . . .

Created a cathode ray tube (also called a Crookes Tube)

This was the precursor to the TV tube - Thank you Crookes!!!

JJ Thomson

1856-1940

British physicist

continued Crookes work

Experiment: applied an electric field which bent the light inside the tube back to a straight path and was able to measure this field.

Thomson’s Outcomes

discovered the electron

used the strength of the field was able to determine the charge-to-mass ratio of this negative particle beam - now so named

determined the existence of the proton (because of neutrality)

Thomson’s Model

Plum Pudding Model

Negatively charged electrons embedded in a positive mass of proton”ness”

JJ’s Pros & Cons

Pro: did explain some of the electrical nature of matter.

Con: did not say anything about the number of charged particles or their arrangement.

Won NOBEL PRIZE in 1906

Robert Millikan

1868-1953

US Physicist

NOBEL PRIZE 1932

Oil Drop Experiment

Millikan’s Outcomes

determined the charge on a single electron

using the charge-to-mass ratio from Thomson, many scientists determined the mass of an electron

William Roentgen

1845-1923

Discovered x-rays

killed his wife

Henri Becquerel

1852-1908

French scientist

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

serendipity

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 product of radioactivity

1896 - credited with the discovery of radioactivity

Marie & Pierre Curie

French physicists

intrigued with Becquerel’s work

studied radioactivity

Marie discovered POLONIUM

Ernest Rutherford1871-1937

New Zealand

discovered the nucleus of the atom

Planetary Motion Model

Gold Foil Experiment Gold Foil Experiment - 2A beam of alpha particles was directed at a thin sheet of gold foil with a fluorescent screen around it.

Most of the alpha particles went straight through it - or had very little deflection.

A few were deflected back at very sharp angles.

A 15-inch shell fired at a piece of tissue paper and it came back and hit you.

Rutherford concludes...

Atoms have almost all of their mass concentrated in a very small positively charged region = the nucleus

The nucleus is surrounded by electrons which are at a relatively large distance.

Why did the electrons not crash into the nucleus in Rutherford’s model?

Planetary Motion Model

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

The nucleus held (only) protons (for the time being).

Hans Geiger

Student of Rutherford

He was one of the people who had to spend long hours (in mostly darkness) counting the particles that hit the screen.

Created the first electrical particle counter - The Geiger counter

Geiger’s contributions

Determined that alpha particles were made up of 2 protons and 2 neutrons. (1908)

This gives an alpha particle a mass number of ??? and a nuclear charge of ???.

Irene Curie-Joliot& Frederic Joliot

Bombarded beryllium with alpha particles - which created a beam of energy that had great power. It was able to penetrate metals.

They thought it was a new form of energy; they thought it was gamma rays.

James Chadwick

1891-1974

British physicist and chemist

Found the Curie-Joliot beam of energy was unaffected by magnetic fields - meaning it must be neutral.

Particles moved at 1/10 the speed of light - so it was NOT radiant energy - it must be PARTICLES

Chadwick’s Particles

The new particles had approximately the same mass as a proton - but these new particles had NO CHARGE.

He is credited with the discovery of the neutron.

After neutrons were discovered, isotopes of various elements were discovered.

Neils Bohr Bohr & Einstein

Bohr continued with energy calculations

and he further developed the

planetary motion model to include neutrons, and

specific energies of electrons in a certain path.