Evolution of the Atomic Theory

Greek Philosophy (400 BC)

• Democritus• “All matter is made of atoms”• Philosophy is knowledge gained

only by observation and logic, not by experiment

John Dalton (1803)

• Dalton’s Theory • Atoms are indivisible and indestructible• All atoms of the same element are the

same• Atoms of different elements are different• Atoms combine in simple whole number

ratios to form compounds• Billiard ball

J. J. Thomson (1880)• Used a cathode ray tube• Determined the particles in the cathode

ray tube to be a stream of negative particles

• Determined that the atom is composed of smaller particles

• Discovered the electron• Robert Millikan determined the charge

and mass of an electron• Blueberry muffin model

Robert Millikan (1910)

• Oil Drop Experiment• Determined charge of electrons by

suspending a drop of oil between magnets

• Combined with Thomson’s experiments, was able to determine mass of electron.

Ernest Rutherford (1909)

• Shot alpha particles at gold foil

Ernest Rutherford (1909)

• Surprised to find that although most particles traveled straight through the foil, some were deflected

• Discovered a positive center that he called the nucleus

• The atom is mostly empty space

Niels Bohr (1913)

• Observed different line spectrums for every element

• Calculated the line spectrum for hydrogen

• Postulated that electrons travel in orbits around the nucleus

• Bohr-ing

Erwin Schrödinger (1926)• Used calculus to

figure out probability of locating electron.

• Mathematical graphs displayed “regions of probability” for location of electrons called “ORBITALS”

James Chadwick (1932)

• Bombarded a beryllium atom with alpha particles

• Discovered that protons and neutrons are in the nucleus

p+

no