The Idea of the Atom

Democritus (460-370 B.C.) Greek philosopher Believed the world was made of two things – empty space and tiny particles Called the tiny particles “atomos”- meaning indivisible

John Dalton (1802) 4 Postulates: 1. Each element is composed of extremely small indivisible particles (same model as Democritus) 2. Determined that all atoms of a given element are identical to each other, but they differ from those of any other element. 3. Law of Conservation of Matter= matter is not created nor destroyed in a chemical reaction 4. Law of Definite Proportions= A given compound always has the same ratio and type of atoms. Ex: water H2O is always 2Hydrogens &1 Oxygen

JJ Thomson (1904) Plum Pudding Model (looks more like a chocolate chip cookie, with negative “chips” and positive dough) Discovered electrons Experiment with cathode ray tube Particles move toward positive end, therefore it must contain negatively charged particles Disproved that the atom was indivisible (it could be broken into parts)

Cathode Ray Tube http://www.aip.org/history/electron/jjappara.htm

Robert Millikan (1911) Oil drop Experiment Able to stop a drop of oil in mid air using magnetic field Could then measure the charge on an electron & calculated the mass of an electron!! Won the Nobel Prize in Physics (1923)

Oil drop Experiment What Millikan did was to put a charge on a tiny drop of oil, and measure how strong an applied electric field had to be in order to stop the oil drop from falling. Since he was able to work out the mass of the oil drop, and he could calculate the force of gravity on one drop, he could then determine the electric charge that the drop must have. By varying the charge on different drops, he noticed that the charge was always a multiple of -1.6 x 10 -19 C, the charge on a single electron. This meant that it was electrons carrying this unit charge.

1911-Rutherford New Atomic Theory Discovered that the atom was mostly empty space with a dense positively charged nucleus Gold Foil Experiment Disproved the Plum Pudding Model (btw he was a grad student of Thomson)

Rutherford’s Experiment http://micro.magnet.fsu.edu/electromag/java/rutherford/

Thomson versus Rutherford

Niels Bohr (1913) Planetary Atomic Model Electrons moved in an orbit Each orbit had a definite energy Neils Bohr chillin with his buddy Albert

Electron Cloud Model The model now is adapted from Bohr’s model Electrons do not follow paths The electron “cloud” shows where the electron is most likely to be Where you have a 90% chance of finding the electron

Heisenberg (1927) Concluded that the only way to know where an electron is, is to hit it with a photon and by doing so you would change it’s location. Known as “Heisenberg’s Uncertainty Principle”

Louise deBroglie- 1924 Dual Nature of Matter Determined that electrons travel as waves. Known as “Matter Waves” Allowed us to build electron microscopes which can see large atoms

1926- Schrodinger Developed a mathematical equation to explain the wave-particle duality Began the field of Quantum Mechanics (aka Quantum Physics)

Today: Quantum Mechanical Model Developed from Heisenberg, de Broglie & Schrodinger

James Chadwick Discovered the 3rd and final subatomic particle-neutron Has no charge but does have the same mass as a proton BTW- He was a POW in WWI, when released he returned to study under Rutherford

History of Atomic Models Dalton 1803 Thompson 1897 “plum pudding” Rutherford 1909 Bohr 1913 “planetary model” Quantum Mechanical Model Electron-cloud model

Atomic Theory Timeline http://www.youtube.com/watch?v=5giHXP7-3eE 1803 1897 1909 1913 1940’s 1750 1800 1850 1900 1950 James Chadwick John Dalton J.J. Thompson Antoine LaVoisier Ernest Rutherford Niels Bohr Joseph Proust Robert Milliken 1770 - Law of conservation of mass 1897 – Cathode Ray Experiment Neutron discovered 1803 – Law of Multiple proportions 1912 – Gold Foil experiment 1911 - Oil drop experiment 1799 – Law Of Definite Proportions