Democritus of John Dalton Abdera

Ancient Greece - 4th century BC 1766-1844

first suggested the existence of tiny Eaglesfield, England fundamental particles that make up matter. Father of modern atomic theory

atoms = indestructible Pro: advance in the description and nature of did not agree with the “current” sci theory - chemical reactions. but did not explain chemical behavior - and Con: did not hold up when subatomic lacked experimental support. particles were disovered

Dalton’s Atomic Theory Dalton’s Atom

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

2.All matter is composed of combinations of The atom is a solid these atoms. indestructible sphere. 3.Atoms of different elements are different.

4.Atoms of the same element have the same size, mass, and form. William Crookes’ Tube Crookes

1832-1919

British physicist investigated why gases glow

Crookes’ Experiment Crookes’ Outcomes

It was a beam of particles?

How did he know? used a partially evacuated tube with a low pressure gas and ran a current through it Determined the particles had a negative charge - due to . . . power on: a green glow was emitted as a beam of light ran across the tube Created a cathode ray tube (also called a If the charge on the plates as switched . . . Crookes Tube) This was the precursor to the TV tube - Thank you Crookes!!! JJ Thomson Thomson’s Outcomes

1856-1940 discovered the electron British physicist used the strength of the field was able to continued Crookes work determine the charge-to-mass ratio of this negative particle beam - now so named Experiment: applied an electric field which bent the light inside the tube back to a determined the existence of the proton straight path and was able to measure this (because of neutrality) field.

Thomson’s Model JJ’s Pros & Cons

Pro: did explain some of the electrical nature Plum Pudding Model of matter. Negatively charged electrons embedded Con: did not say anything about the number in a positive mass of of charged particles or their arrangement. proton”ness” Won NOBEL PRIZE in 1906 Oil Drop Experiment Robert Millikan

1868-1953

US Physicist

NOBEL PRIZE 1932

Millikan’s Outcomes William Roentgen

determined the charge on a single electron 1845-1923

using the charge-to-mass ratio from Discovered x-rays Thomson, many scientists determined the mass of an electron killed his wife Henri Becquerel Becquerel’s outcomes

1852-1908 concluded something in the sample itself must be capable of “fogging” the plates; the French scientist sample must give off some kind of ray without the influence of the sun Left a sample of uranium ore in a drawer with glass photo plates and there was rays were the product of radioactivity fogging on the plates 1896 - credited with the discovery of serendipity radioactivity

Marie & Pierre Curie Ernest Rutherford French physicists 1871-1937 intrigued with Becquerel’s work New Zealand

studied radioactivity discovered the nucleus of the atom

Marie discovered Planetary Motion Model POLONIUM Gold Foil Experiment Gold Foil Experiment - 2 A 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... Planetary Motion Model

Atoms have almost all of their mass Electrons orbit concentrated in a very small positively around the charged region = the nucleus nucleus like planets orbit The nucleus is surrounded by electrons which around the sun. are at a relatively large distance. The nucleus held Why did the electrons not crash into the (only) protons (for nucleus in Rutherford’s model? the time being). Created the first electrical particle Hans Geiger counter - The Geiger counter

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.

Geiger’s contributions Irene Curie-Joliot & Frederic Joliot

Determined that alpha particles were made Bombarded beryllium with alpha particles - up of 2 protons and 2 neutrons. (1908) which created a beam of energy that had great power. It was able to penetrate This gives an alpha particle a mass number metals. of ??? and a nuclear charge of ???. They thought it was a new form of energy; they thought it was gamma rays. James Chadwick Chadwick’s Particles

1891-1974 The new particles had approximately the same mass as a proton - but these new British physicist and chemist particles had NO CHARGE.

Found the Curie-Joliot beam of energy was He is credited with the discovery of the unaffected by magnetic fields - meaning it neutron. must be neutral.

Particles moved at 1/10 the speed of light - so it was NOT radiant energy - it must be After neutrons were discovered, isotopes of PARTICLES various elements were discovered.

Bohr & Neils 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.