Atomic Theory Past and Present: pieces of a puzzle
The First Atomic Hypothesis Democritus (460 370 BC): Greek philosopher Speculated that matter is composed of atoms which move through empty space Atoms are solid, homogeneous, indestructible and indivisible Different types of atoms have different sizes and shapes. Atomos = indivisible
The Wrong Direction Aristotle (384 322 BC): Aristotle rejected Democritus ideas and said empty space could not exist. Instead, he believed that matter is only made of earth, fire, air and water. Very influential philosopher of the time whose ideas were accepted. His denial of the existence of atoms went unchallenged for 2000 years!! People quit looking for atoms for 2 millenia.
The Early Atomic Hypotheses Boyle (1627-1691): In 1650, without proof, Boyle stated that matter was composed of atoms. He discovered a gas law. Newton (1642-1727): In 1670, without proof, stated that matter was composed of atoms. He discovered g and calculus.
Law of Conservation of Mass Lavoisier (1760): Founded modern chemistry Discovered 33 elements and organized them into a periodic table Discovered combustion Discovered the Law of Conservation of Mass Matter cannot be created nor destroyed. Mass of reactants equals the mass of products.
Law of Conservation of Mass Mass is conserved during chemical reactions!
Law of Conservation of Mass In a chemical reaction atoms separate, combine or rearrange, but, atoms are not created, destroyed or divided in the process. The number of atoms of each type is the same before or after the reaction. (see previous slide) The mass of the reactants is the same as the mass of the products.
John Dalton s Atomic Theory Dalton (1766-1844): John Dalton, a practicing Quaker left school at age 11 and returned at age 12 as a teacher in England In 1803 after studying the works of Democritus and Boyle and others, and performing numerous experiments, he proposed his atomic theory It was the first theory based on scientific evidence.
John Dalton s Atomic Theory 1. Matter is composed of extremely small particles called atoms 2. Atoms are indivisible and indestructible 3. Atoms of a given element are identical in size, mass and chemical properties 4. Atoms of a specific element are different from those of another element 5. Different atoms combine in simple whole-number ratios to form compounds 6. In a chemical reaction atoms are separated, combined or rearranged
Law of Multiple Proportions: when 2 elements combine to form different compounds in small whole number ratios (predicted by Dalton s theory) (NH 4 ) 2 Cr 2 O 7 (orange) Cr 2 O 3 (green) CrO 3 (red) 2.167 g : 1.000 g 1.083 g : 1.000 g 2.167 g : 1.083 g 2 : 1
Scientists still thought the atom was the smallest particle of matter for nearly 100 years until.
Discovery of Electrons Thomson (1856-1940): In 1897, Thomson conducted the Cathode Ray Tube Experiment (CRT). Thomson discovered the charge to mass ratio of an electron (1.76 x10 8 C/g) Thomson proposed the Plum Pudding Model of the atom
Cathode Ray Tube
The Gold Foil Experiment The nucleus is discovered. Rutherford (1871-1937): In 1911, Rutherford shot alpha particles from Thorium at a thin layer of gold foil. Most went straight through Some deflected a little A few were deflected a lot (cannon ball off TP) Conclusion: Atoms are mostly empty space Atoms have a very small dense nucleus that is positively charged.
The Gold Foil Experiment
The Electron s Charge is Discovered Millikan (1868-1953): In 1916, using the Oil Drop Apparatus Millikan discovered the charge of an electron. Robert Millikan -1.6 x10-19 C / e-
Millikan Oil Drop Experiment
Neutron Discovered Chadwick (1932): Discovered the neutron by bombarding Li, Be, and B with alpha particles. 4 2 α ---------------> Li Target ------------> 1 0n The neutron has no charge and has a mass 1840 times heavier than that of an electron Neutron Mass = 1.675 x10-24 g Neutron Charge = 0 + -
Niels Bohr s Atom Electrons orbit the nucleus in orbits, like a solar system; multiple energy levels Planetary Model
Quantum Mechanical Model like a shot gun blast. Erwin Schrödinger 1887-1961 Born in Vienna, Austria University of Berlin professor Nobel Prize 1933 Based on waves of light and probability.
Modern Atomic Structure Electron Cloud: Electrons located outside the nucleus Volume of atom is mostly empty space Most atoms radii = 1 x10-10 m = 1Ǻ Nucleus: Protons and neutrons are located in the nucleus. Most of the mass of the atom is in the nucleus. Nucleus size = 1 x10-15 m
Atomic Models
Atomic Size So how big is it? A cubic cm of air (size of a sugar cube) has 4.5 x 10^19 atoms! 45,000,000,000,000,000,000 atoms How many atoms would it take to fill a universe? http://www.youtube.com/watch?v=0fkbhvdju y0&feature=player_embedded
IBM s Scanning Tunneling Electron Microscope
Quarks, Leptons, Hadrons.. No model can accurately represent every known aspect of the atom! Youtube video: Exploring the Atomic Composition of Steel