Ernest Rutherford (1908) an English physicist bombarded a thin sheet of gold foil with alpha particles, expecting most of the particles

to pass through unaffected and a few being deflected by their interaction with the negatively charged particles. However, much to his

amazement, he discovered that some of the particles bounced back. He concluded that an atom must be mostly empty space with a

small dense positively charged particle in the center surrounded by negatively charged electrons. This made Rutherford the first to

observe protons.

Democritus (BC 400) and the “Atomists” debate the “four elements (fire, water, earth,

and air). They conclude that an object could not be cut in half again and again

indefinitely. They decided that sooner or later the object would become so small that it

could not be divided again. They called this indivisible invisible particle an “Atom”.

(Atom is the Greek work meaning “not to be cut” or “indivisible”).

John Dalton (1803), an English meteorologist, used his knowledge of gases in the

atmosphere as a model for the atom. He is given credit for proposing the first “Atomic

Theory”:

• All elements are composed of atoms.

• Atoms are indivisible, invisible particles.

• Atoms of the same element are the same.

• Atoms of different elements are different.

• Compounds are formed by two or more atoms joined together.

As he studied the passage of electricity through a gas in a cathode ray tube, J. Jospeh

Thomson (1897) an English scientist discovered that the atom has negatively charged

particles. He called them “corpuscles”. (Today we know them as electrons.) He

concluded that there would also have to be positively charged particles to balance out the

negatively charged particles in the atom, but could never find them. As a result, he

purposed the “Plumb Pudding” model of atoms, with electrons in a positively charged

“sea” much like plumbs in pudding.

Niels Bohr (1913) a Danish Scientist proposed an improved model of the atom

wherein the electrons were much like the planets in our solar system orbiting

around the nucleus.

James Chadwick (1932) is credited with the discovery of neutrons.

Werner Heisenberg (1926), a German Physicist, showed that the position of an electron can never be precisely known. He purposed that there were regions about the nucleus called “orbitals” where the

electrons would most likely be found. These ideas led to the introduction of “wave mechanics”

Today:

Quarks, Pi masons, . . .????? Who knows, perhaps the textbooks of the future will include your name and picture here along with the neat discoveries that you will make.