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Standard 5: Atomic Theory How it has changed over time Democritus Greek philosopher Decided that all matter was made up of tiny indivisible particles Greek word for indivisible = “atomos” First person to use the term atom Idea faded as Aristotle’s ideas were more popular (matter is divine, spiritual) John Dalton Scientist – performed experiments After the renaissance, Democritus’ ideas of atoms gained popularity Dalton studied many ideas about atoms and developed the first atomic theory Dalton’s Atomic Theory 1. 2. 3. 4. All matter is made of atoms. Atoms are indivisible. Atoms of the same element are identical, but atoms of different elements are different. Atoms of different elements unite to form compounds. J. J. Thomson Discovered the electron while studying cathode ray tubes. Why does the beam of electrons move upward? Downward? He realized that if there was an electron in the atom, there must be a proton to cancel out the negative charge. J. J. Thomson’s Plum Pudding Model The atom is not a solid sphere, but has electrons floating around in a positive material. Plum Pudding Protons and Electrons Proton = positive particle (+) Electron = negative particle (-) # of Protons = # of Electrons NEUTRAL atom Atomic Number = # of protons in an atom Rutherford’s Gold Foil Experiment Video: Through the Wormhole: Episode 7 Rutherford’s Planetary Model Rutherford discovers the NUCLEUS -- tiny, positive center of an atom 1. 2. 3. Proton = positively charged particle Electron = negatively charged particle Neutron = particle with no charge Electrons are found orbiting the nucleus. NUCLEUS protons + neutrons located inside Calculating PEN numbers Atomic Number = # of protons in the nucleus of an atom # of protons = # of electrons Element Sulfur S Iron Fe Atomic # # Protons # Electrons Calculating PEN numbers Mass Number # of protons # of neutrons in the nucleus of an atom MUST round off the atomic mass found on the periodic table red number Magnesium Boron Neon Writing Symbols Mass Number Atomic Number 197 79 Au X 16 8 O Calculating PEN numbers Atomic # = PROTONS protons = # of ELECTRONS # of NEUTRONS = mass # – atomic # P= ___ K E= ___ N= ___ Bohr Models Bohr model of the atom electrons travel in rings around the nucleus 288 NUCLEUS Niels Bohr 1885 - 1962 Parts of a Bohr Model Isotopes Isotopes – atoms of the same element same number of protons (same atomic number) different number of neutrons (diff. mass number) There are three isotopes of carbon: Carbon – 12 12 6 Carbon – 13 C 6 Carbon – 14 C C 6 P P P E E E N N N