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HISTORY OF THE ATOM AND ATOMIC THEORY ATOMS • The smallest part of an element that retains the properties of that element DEMOCRITUS • Greek philosopher • Came up with the word “atom” • Thought atoms were indivisible (couldn’t be broken down any further) JOHN DALTON • All atoms of an element are exactly alike. • Atoms of different elements combine to form compounds. • Law of Definite Proportions - those compounds always contain elements in the exact same proportions –Ex.; water is always H2O JJ Thomson • Discovered atoms were NOT indivisible… they could be divided into smaller parts • Discovered electrons – negatively charged particles within the atom • Plum-Pudding Model – said the electrons were spread throughout the atom ERNEST RUTHERFORD • Gold foil experiment – helped revise Thomson’s Plum-Pudding model – positively charged particles aimed at a thin sheet of gold foil. – thought most particles would travel straight through the foil, a few would be slightly deflected. – most went through, but some were deflected by a large amount – this indicated there was a positive charge in the center of the atom, and the center was small compared to the size of the atom GOLD-FOIL EXPERIMENT RUTHERFORD’S MODEL OF THE ATOM • Nucleus – center of the atom containing protons (positively charged) and neutrons (no charge) • Electrons – negatively charged particles that orbit around the nucleus NIELS BOHR • Suggested electrons are in certain energy levels surrounding the nucleus and they gain energy to move to higher levels or lose energy to move to lower levels MODERN ATOMIC THEORY • Nucleus of an atom is small, dense & in the center with protons and neutrons. • Electrons don’t occupy energy levels • Electrons behave more like waves than particles • It’s impossible to pinpoint the exact location of an electron • Orbital – region of space around the nucleus where an electron is likely to be