The History of Atomic Theory

Democritus Greek philosopher 2400 years ago The Atom Could matter be divided into smaller and smaller pieces forever? Or was there a limit to the number of times a piece of matter could be divided?

Atomos Matter could NOT be divided into smaller and smaller pieces forever This piece would be indivisible. “atomos” =“not to be cut.”

Atomos   Atoms= small, hard particles that were all made of the same material   Different shapes and sizes.   Atoms were infinite in number, always moving and capable of joining together.

Aristotle Same time period Denounced the idea of atomos Earth, Water, Air, and Fire

Alchemists Middle Ages Worked on turning ordinary metals into gold Derived modern chem

John Dalton English chemist Early 1800’s Revisited the particle theory Laid the groundwork for Modern theory Father of the Atom

Dalton’s Theory 1. All matter  small particles a.k.a atoms 2. Atoms of the same element are exactly alike. Atoms of different elements are different. 3. Atoms cannot be divided, created, nor destroyed

Dalton’s Theory 4. Atoms of different elements can combine to form compounds 5. In chemical reactions, atoms are combined, separated, or rearranged Known as the “Billiard Ball Model”

J. J. Thomson English scientist 1897 The electron The first to believe an atom is made of even smaller particles.

Thomson Model Atoms were made from a positively charged substance with negatively charged electrons scattered A.k.a: the “Plum Pudding Model”

Thomson Model: Cathode Ray Thomson studied the passage of an electric current through a gas. As the current passed through the gas, it gave off rays of negatively charged particles.

Robert A. Millikan American physicist 1909 Determined the mass of an electron relative to the atom

Millikan’s: Oil Drop Experiment Mist of fine oil droplets are dropped into the chamber Some of the droplets were negatively charged No electricity  electrons fell through: gravity is only acting force

Millikan’s Oil Drop Experiment Voltage adjusted so down force = up force Oil drop suspended between plates The charge on the droplet was calculated Mass of electron = charge of electron: charge-to-mass ratio of electron Mass = 1/2000 of the Hydrogen atom

Ernest B. Rutherford English physicist English physicist Gold Foil Experiment Gold Foil Experiment

Gold Foil Experiment: Results Most of the positively charged “bullets” passed right through the gold atoms in the sheet of gold foil without changing course at all. Some of the positively charged “bullets,” however, did bounce away from the gold sheet as if they had hit something solid.

What the Gold Foil Experiment Proves

Niels Bohr Danish scientist 1913 Solar System Model

Bohr Model According to Bohr’s atomic model, electrons move in definite orbits around the nucleus, much like planets circle the sun. These orbits are located at certain distances from the nucleus based on relative energies.

1932 NeutronNeutron: There was unaccounted mass so there had to be another subatomic particle Isotopes: Atoms of the same element (identical protons) but different numbers of neutrons and therefore different masses James Chadwick

Electron Cloud Model

The Electron Cloud Model Heisenburg Uncertainty Principle: cannot know speed and location of electron at same time Electrons in organized orbitals Electrons travel so fast that they seem to form a “cloud” around the nucleus Location of electron depends on its energy Light spectrum

Electron Cloud: Electrons: low NRG are closest to nucleus Electrons: high NRG are further from nucleus

Historical Models

IndivisibleElectronNucleusOrbitElectron Cloud Greek X Dalton X Thomson X Rutherford X X Bohr X X X Wave X X X