History of the Atom
History of the atom Not the history of atom, but the idea of the atom. Original idea Ancient Greece (400 B.C.) Democritus - Greek philosopher.
History of Atom Looked at beach Made of sand Cut sand - smaller sand Smallest possible piece? Atomos - not to be cut
Another Greek Aristotle - Famous philosopher All substances are made of 4 elements Fire - Hot Air - light Earth - cool, heavy Water - wet Blend these in different proportions to get all substances
Who Was Right? Did not experiment. Greeks settled disagreements by argument. Aristotle was a better debater - He won. His ideas carried through middle ages. Alchemists tried to change lead to gold.
Who’s Next? 1803 - John Dalton- England. Teacher- summarized results of his experiments and those of others. Elements substances that can’t be broken down In Dalton’s Atomic Theory Combined idea of elements with that of atoms.
Dalton’s Atomic Theory All matter is made of tiny indivisible particles called atoms. Atoms cannot be destroyed. In chemical reactions, the atoms rearrange, but do not break apart. All atoms of the same element have the same properties and identical masses. Atoms of different elements have different properties and masses. When atoms of one element combine with the atoms of another element, they always combine in the same proportional ratio.
Dalton’s Theories Supported Law of Definite Proportion (Chemical Composition) Law of Conservation of Mass Law of Multiple Proportions
Law of Definite Proportions (Law of Chemical Composition) Each compound has a specific ratio of elements. It is a ratio by mass. Water is always 8 grams of oxygen for each gram of hydrogen.
Law of Conservation of Mass Mass is neither created or destroyed in a chemical reaction.
Law of Multiple Proportions If two elements form more than one compound, the ratio of the second element that combines with 1 gram of the first element in each, is a simple whole number. The ratio of the ratios is a whole number.
What? Water is 8 grams of oxygen per gram of hydrogen. Hydrogen peroxide is 16 grams of oxygen per gram of hydrogen. 16 to 8 is a 2 to 1 ratio. True because you have to add a whole atom, you can’t add a piece of an atom.
Parts of Atoms J. J. Thomson - English physicist. 1897 Made a piece of equipment called a cathode ray tube. It is a vacuum tube - all the air has been pumped out. A limited amount of other gases are put in
Thomson’s Experiment Voltage source - + Metal Disks
- + Thomson’s Experiment Voltage source Passing an electric current makes a beam appear to move from the negative to the positive end
Thomson’s Experiment Voltage source + - By adding an electric field
Thomson’s Experiment Voltage source + - By adding an electric field By adding an electric field he found that the moving pieces were negative
Thomson’s Experiment Used many different metals and gases Beam was always the same Ray was deflected if a magnetic or electrical field were brought near Found that cathode ray would bend toward the positive pole The cathode rays were negatively charged; were called electrons
Thomsom’s Model Discovered the electron. Model shows negatively charged electrons embedded into an atom filled with positively charged material Said the atom was like plum pudding.
Millikan’s Experiment - 1909 Atomizer - + Metal Plates Oil Microscope
Millikan’s Experiment Atomizer Oil droplets - + Oil Microscope
Millikan’s Experiment X-rays X-rays give some drops a charge by knocking off electrons
Millikan’s Experiment + -
Millikan’s Experiment - - + + They put an electric charge on the plates
Millikan’s Experiment - - + + Some drops would hover
Millikan’s Experiment - - - - - - - + Some drops would hover + + + + + + +
Millikan’s Experiment - - + + From the mass of the drop and the charge on the plates, he calculated the charge on an electron
Rutherford’s Experiment Ernest Rutherford English physicist. (1911) Believed the plum pudding model of the atom was correct. Wanted to see how big they are. Used radioactivity. Alpha particles - positively charged pieces given off by uranium. Shot them at gold foil which can be made a few atoms thick.
Rutherford’s experiment When the alpha particles hit a florescent screen, it glows.
Flourescent Screen Lead block Uranium Gold Foil
He Expected The alpha particles to pass through without changing direction very much. Because… The negative charges were spread out evenly. Alone they were not enough to stop the alpha particles.
What he expected
Because
Because, he thought the mass was evenly distributed in the atom
Because, he thought the mass was evenly distributed in the atom
What he got
How he explained it Atom is mostly empty. Small dense, positive piece at center. Alpha particles are deflected by it if they get close enough. +
+
Density and the Atom Since most of the particles went through, it was mostly empty. Because the pieces turned so much, the positive pieces were heavy. Small volume, big mass, big density. This small dense positive area is the nucleus.
Niels Bohr - 1922 Applied quantum theory to Rutherford's atomic structure by assuming that electrons travel in stationary orbits around the nucleus. Often referred to as the “Planetary Model” This led to the calculation of possible energy levels for these orbits and the postulation that the emission of light occurs when an electron moves into a lower energy orbit.
Louis de Broglie - 1923 Thomson’s experiments demonstrated that electrons act like particles that have mass. de Broglie pointed out that the behavior of electrons was similar to the behavior of waves. (like light) Frequencies could correspond to the specific energy levels associated with the Bohr model.
Modern View The atom is mostly empty space. Two regions. Nucleus- protons and neutrons. Electron cloud- region where electrons are located in areas of high probability called orbitals
Other pieces Proton - positively charged pieces 1840 times heavier than the electron. (Discovered in 1914 by Henry Moseley) Neutron - no charge but the same mass as a proton. (Discovered by James Chadwick in 1932) Where are the pieces?