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.
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