ATOMIC THEORY TIMELINE

DEMOCRITUS 400 BC

ATOMOS THEORY

• First person to come up with the idea of atoms
• All matter consists of invisible particles aka atoms
• Greek philosopher who began the search for a description of matter more than 2400 yrs ago
• His theory was that matter could not be divided into smaller and smaller pieces forever
• He named the smallest piece of matter atomos, which means not to be cut

ARISTOTLE 300 BC

4 ELEMENT THEORY

• Aristotle favored the four element approach to the matter of nature
• Their ideas held sway because of their eminence as philosophers
• The atomos idea was buried for approximately 2000 years
• At the time, this was a more respected (but ultimately wrong) theory
• Earth , wind , fire , and water were the four elements.

DALTON 1803

BILLIARD BALL MODEL

• Performed a number of experiments leading to his acceptance of atoms
• Atoms of the same element are exactly alike
• Compounds are formed by the joining of atoms of two or more elements.
• He deduced that all elements are composed of atoms.
• Said the atom looked like a billiard ball

THOMSON 1897

PLUM PUDDING MODEL

• Provided the first hint that an atom is made of even smaller particles
• Thomson studied the passage of an electric current through a gas
• As the current passed through the gas, it gave off rays of - charged atoms
• Atoms of the gas were uncharged
• Thomson concluded that the negative charges came from within the atom

RUTHERFORD 1908

GOLD FOIL EXPERIMENT

• Firing a stream of tiny positively charged particles at a thin sheet of gold foil
• Positively charged “bullets” passed right through the gold atoms in the sheet without changing course at all.
• Some bounced away from the gold sheet as if they had hit something solid
• He knew that positive charges repel positive charges
• Rutherford concluded that an atom had a small, dense, + charged center that repelled his + charged bullets.

MILLIKAN 1908

OIL DROP EXPERIMENT

• Experiment performed that determined the size of the charge on an electron
• Determined there was a smallest 'unit' charge, or that charge is 'quantized'
• Made of rise and fall times of oil drops lit by light of a helium neon laser
• Charge on the oil drops could be determined when the drops were held at equilibrium
• Conducted the the oil drop experiment

CHADWICK 1932

NEUTRON DISCOVERY

• Discovered protons had actually been hit by neutrons
• In 1935, James Chadwick received the Nobel Prize in Physics for his discovery of the neutron.
• The Joliot-Curies believed they'd managed to eject protons from a sample of wax using gamma rays.
• Chadwick, and some others believed in the possibility that particles with no charge could be in the nucleus.