Not the history of the atom, but the idea of the atom The atom was not discovered until recently Original Idea Ancient Greece (400 BC) Proposed by lesser-known scientists They looked at a beach made of sand If you cut you get smaller sand What is the smallest possible piece? Atomos not to be cut
Aristotle Famous Philosopher All substances are made of four elements: Fire Hot, dry Air Light Earth Cool, heavy Water - Wet These are blended in different proportions to get all substances
Greek society was class-based It was thought to be beneath famous scientists to work with their hands Aristotle did no experiments Greeks settled disagreements by argument/debate Aristotle was more famous he usually won His ideas carried through the Middle Ages
Late 1700 s John Dalton (England) He was a teacher, NOT a scientist He summarized the results of his experiments and those of others He came up with Dalton s Atomic Theory This combined the ideas of elements with those of atoms Dalton s Atomic Theory All matter is made of tiny indivisible particles called atoms Atoms of the same element are identical, those of different atoms are different Atoms of different elements combined in whole-number ratios to form compounds Chemical reactions involve the rearrangement of atoms. No new atoms are created or destroyed
Law of Definite Proportions Each compound has a specific ratio of elements This ratio is by mass For example, water is always 8 grams of oxygen for each gram of hydrogen Law of Multiple Proportions If two elements form more than one compound, the ratio of the second element that combines with one gram of the first element in each is a simple whole number
J. J. Thomson English Physicist (1897) Made a piece of equipment called a cathode ray tube A cathode ray tube is simply a vacuum tube - Voltage source + Vacuum tube Metal Disks
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- Voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end
- Voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end
- Voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end
- Voltage source + Passing an electric current makes a beam appear to move from the negative to the positive end
Voltage source + - By adding an electric field
Voltage source + - By adding an electric field
Voltage source + - By adding an electric field
Voltage source + - By adding an electric field
Voltage source + - By adding an electric field
Voltage source + - By adding an electric field
Voltage source + By adding an electric field he found that the moving pieces were negative -
Found the Electron But he couldn t find anything positive Said the atom was like plum pudding A bunch of positive stuff, with the electrons able to be removed
Proton Positively-charged pieces 1840 times heaver than the electron Neutron No charge but the same mass as a proton But where are they located?
Earnest Rutherford English Physicist (1910) Believed in the Plum Pudding Model of the Atom (Thomson) Wanted to know how large an atom was He used Radioactivity Alpha Particles positively-charged particles given off by uranium (they are simply the nucleus of a helium atom) Shot Alpha Particles at gold foil that could be made a few atoms thick He expected the alpha particles to pass through without changing direction much Why? The positive charges were spread out evenly (the pudding). Alone they were not enough to stop the alpha particles
Lead block Uranium Fluorescent Screen Gold Foil When alpha particles strike a fluorescent screen, it glows.
Because, he thought the mass was evenly distributed in the atom
Atom is mostly empty Small dense, positive piece at center Alpha particles are deflected by nucleus if they got close enough +
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The atom is mostly empty space Two regions: Nucleus protons & neutrons Electron Cloud where you might find an electron
Since most of the particles went through, the atom is mostly empty Because the alpha particles turned so much, the positive pieces were heavy Small volume, big mass, BIG density The small, dense positive area is called the nucleus
Rutherford s Model of the Atom atomic radius ~ 100 pm = 1 x 10-10 m nuclear radius ~ 5 x 10-3 pm = 5 x 10-15 m If the atom is the Houston Astrodome, then the nucleus is a marble on the 50-yard line.