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History of Atoms Use the information on each slide to fill in the blanks on the student notes. History of Atoms Democritus Who first thought of the atom? The Greek philosopher Democritus (about 400 BC) – he thought if you cut an object over and over and over again, there would eventually be a piece that you couldn’t cut anymore. He called that piece an atom. In Greek, atom means “not to be cut” or “indivisible”. Why is this a big deal? Because … Democritus by Agostino Carracci (15571602) History of Atoms Aristotle Aristotle was another philosopher and he believed that everything was made of four elements: wind, water, earth and fire. His was the accepted explanation, because no one could see atoms! Water. Earth. Fire. Air. Long ago, the four nations lived together in harmony. Then, everything changed when the Fire Nation attacked … History of Atoms John Dalton The Atomic Theory John Dalton was an English Scientist. In 1803 he proposed the atomic theory that states: -Atoms cannot be divided -Atoms are invisible -All elements are made of atoms -Atoms of the same element are the same -Atoms of different elements are different -Molecules are formed by two or more atoms joined together Ideas about Atoms Democritus’s Deductions • Atoms are small solid objects that cannot be divided, created, or destroyed • Atoms are constantly moving in empty space • Different types of matter are made of different types of atoms. • The properties of atoms determine the properties of matter. Dalton’s Atomic Theory • All matter is made of atoms that cannot be divided, created or destroyed. • During a chemical reaction, atoms of one element cannot be converted into atoms of another element. • Atoms of one element are identical to each other but different from atoms of another element. • Atoms combine in specific ratios. History of Atoms Joseph J. Thomson What are atoms made of? In 1897, a scientist named Joseph J. Thomson was experimenting with gases and electricity and discovered that atoms have negatively and positively charged particles. The negatively charged particles are called electrons and the positively charged particles are called protons. See book, page 84-85 History of Atoms Ernest Rutherford What else are atoms made of? Ernest Rutherford discovered in 1908 that atoms are mostly made of empty space! In the center of the atom is a dense nucleus of protons surrounded by electrons. In fact, if the nucleus of an atom was the size of a pea, the atom would be as big as a football stadium! The electrons would be flying around the outside of the stadium, and the rest of the atom would be empty space! See page 86-88 Atomic Models Thomson “Plum pudding” model Rutherford History of Atoms Niels Bohr What do atoms look like? In 1913, Niels Bohr proposed that the atoms look like a solar system, with the protons in the nucleus acting like the sun, and the electrons flying around the nucleus like planets orbiting the sun. See page 89 Proton Neutron Electron History of Atoms Werner Heisenberg What do atoms look like? In 1926, Werner Heisenberg proposed that electrons don’t really fly all around the atom. He said that electrons had certain spots they floated around in called orbitals (kind of like balloons). History of Atoms James Chadwick The Last Piece In 1932, James Chadwick discovered that atoms also contain neutrons. Neutrons do not have a charge, they are “neutral”. The neutrons and the protons make up the nucleus, or the center of the atom.