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Early Models of the Atom Democritus Said that all things are made of tiny invisible particles called atomos Joseph Louis Proust Law of Constant Composition A given compound always contains the same elements in the same proportion by mass John Dalton • Atomic Theory of Matter – Each element is made of small particles called atoms – All atoms of same element are identical. – Atoms are neither created or destroyed – A given compound has the same number and kind of atoms Atom • Definition- Discovering Atomic Structure • Early scientists thought the atom was hard and round. • Michael Faraday – Said the structure of the atom was related to electricity. • Static electricity – Static means – Electricity comes from the greek word Elekton • Ben Franklin – Kite Experiment • Concluded that there are 2 charges – Also concluded J.J Thomson • Cathode Ray Tube – Simple version of the TV Tube – Cathode had a – Anode had a – Concluded that the cathode ray was made of negative particles – He named the negative particles electrons Robert Millikan • Oil Drop Experiment – Measured the charge of an electron – Found the mass of the electron at 9.11 x 10-28 g Radioactivity • Henri Becquerel – Accidently found radioactivity – Placed uranium on photographic film • Pierre and Marie Curie – Isolated radium and polonium – Given credit for discovering radioactivity Ernest Rutherford • Father of Chemistry • Took over where JJ Thomson left off. – Shot a beam of particles through two opposite charged plates and called the positive particles alpha particle, called the negative particles beta particles, and the neutral substance was gamma waves. • Gold foil experiment – Look at class handout – The reason the alpha particles split -Named the nucleus Models of the Atom • Dalton’s model – Round and hard sphere • Thomson’s model ( Plum Pudding Model) – Negative and positive charges evenly distributed • Rutherford’s model – Positive charges in the nucleus – Negative charges out side the nucleus • Bohr’s model – Nucleus with protons and neutrons inside – Electrons orbiting out side the nucleus Bohr’s Model • • • • Protons = atomic number Electrons= atomic number or protons Neutrons= atomic mass – atomic number Maximum number of electrons per orbit – – – – – – – 1st orbit = 2 2nd orbit= 8 3rd orbit= 8 4th orbit = 18 5th orbit = 18 6th orbit = 32 7th orbit = 32 Henry Moseley • Worked under Rutherford • Found that the identity of the atom comes from the number of positive charges • He is given credit for naming the proton. Ions • Atoms that lose or gain electrons • Positive ions lose electrons • Negative ions gain electrons – this is because of the charge of the electron • Examples Isotopes • Atoms with the same number of protons but different number of neutrons. • Examples Radioactive Decay • Atoms emit one of three kinds of radiation – Alpha equal to a helium atom – Beta equal to an electron – Gamma no particles • Alpha decay – Ra – U + + • Beta decay – I – C + + low penetration mid penetration high penetration