THE ATOM
History Lesson (400 B.C.) Democritus Greek philosopher Belief that all matter is made up of tiny indivisible particles NO EVIDENCE Called this “atom” Greek word – atomos = indivisible
Aristotle (350 B.C.) Believed all matter is made up of four elements Fire Water Air Earth
1800’s John Dalton Developer of atomic theory
Dalton’s Atomic Theory Elements are composed of tiny, indivisible particles called atoms All atoms of an element are identical and have the same properties (proven wrong because elements have atoms with different masses aka isotopes Atoms are indestructible (Law of conservation of mass) Compounds contain atoms in small definite whole-number ratios (Law of definite proportions) Atoms may combine in more than one ratio to form different compounds (Law of multiple proportions)
Law of Conservation of Mass Mass is neither created not destroyed by any means including physical/chemical reactions Can not make matter out of nothing Can not make matter disappear/turn to nothing
Law of Definite Proportions Every chemical compound contains fixed and constant proportions Ex: Water will always be made up of 2 Hydrogen atoms to 1 Oxygen atom. Will always have a 2:1 proportion Ex: NaCl: 39.34% by mass of Na and 60.66% by mass of Cl
Law of Multiple Proportions Two or more different compounds can be composed of the same elements, but in different ratios Ex: CO- Carbon Monoxide (1C:1O) CO2 – Carbon Dioxide (1C:2O) Different compounds = different properties
JJ Thomson Cathode Ray experiment Proved the existence of negatively charged particles (electrons) Cathode (negatively charged plate) and Anode (postively charged plate) are placed inside a vacuum sealed tube. Electricity (contains electrons!) runs through the cathode Electrons get repelled by the cathode/attracted to anode Small hole in anode allows electrons to pass, creating a beam of electrons
Cathode Ray
Cathode Ray Experiment
Plum Pudding Model Hypothesized that atoms were mostly empty space and electrons
Millikan (1909) Oil drop experiment Observing tiny droplets of oil between two electrode plates Oil drop would fall first without any voltage Electric field strength would increase until oil drop was suspended in air Able to determine charge of an electron
Oil Drop Experiment
Oil Drop Experiment
Rutherford (1910) Gold Foil Experiment Alpha particles (positively charged) shot through a sheet of gold Most went through, but some deflected A few bounced back This concluded that atoms have a dense positive charged nucleus that is surrounded by electrons
Gold Foil Experiment
Gold Foil Experiment
Bohr (1913) Provided the planetary model of the atom Electrons circle the nucleus similar to planets around the sun Explains spectral lines for quantized energy and colors/light
Bohr Model
Chadwick (1935) Discrepancy between atomic number (# of protons) and atomic mass Beryllium was bombarded with alpha particles Radiation emitted, thought to be gamma rays Radiation behaved like gamma rays, but different Particle was capable of colliding and knocking away protons Discovery of neutrons, n0 Isotopes Atoms with the same # of protons (same element) but different number of neutrons