2MODELS OF ATOMSDemocritus(400 B.C): said that matter is composed of indestructible particles. He called them “atoms”, a Greek word which means indestructible particle.
3The Early History of Chemistry Before 16th CenturyAlchemy: Attempts (scientific or otherwise) to change cheap metals into gold17th CenturyRobert Boyle: First “chemist” to perform quantitative experiments18th CenturyGeorge Stahl: Phlogiston flows out of a burning material.Joseph Priestley: Discovers oxygen gas, “dephlogisticated air.”
4Law of Conservation of Mass Discovered by Antoine LavoisierMass is neither created nor destroyed
5Law of Definite Proportion (Proust) A given compound always contains exactly the same proportion of elements by mass.Carbon tetrachloride is always 1 atom carbon per 4 atoms chlorine.
6Law of Multiple Proportions (Dalton) When two elements form a series of compounds, the ratios of the masses of the second element that combine with 1 gram of the first element can always be reduced to small whole numbers.The ratio of the masses of oxygen in H2O and H2O2 will be a small whole number (“2”).
8Dalton’s Atomic Theory (1808) Each element is made up of tiny particles called atoms.The atoms of a given element are identical; the atoms of different elements are different in some fundamental way or ways.
9Dalton’s Atomic Theory (continued) Chemical compounds are formed when atoms combine with each other. A given compound always has the same relative numbers and types of atoms.Chemical reactions involve reorganization of the atoms - changes in the way they are bound together. The atoms themselves are not changed in a chemical reaction.
10Early Experiments to Characterize the Atom J. J. Thomson ( ) - postulated the existence of electrons using cathode ray tubes.
12Deflection of Cathode Rays by an Applied Electric Field
13Thompson found electrons were produced from Electrodes of various metals.Therefore, he concluded that all atoms must containelectrons.Since atoms are neutral, he concluded that atoms mustcontain positive charge.
20CONCLUSIONS The atom contains a dense center called the nucleus. The nucleus contains a positive charge and most of the mass of the atom. The nucleus is approximately 100,000 times smaller than the atom.
21CONCLUSIONSThe nucleus is positively charged. The amount of positive charge balances the negative charge of the electrons.The electrons move around in the empty space of the atom surrounding the nucleus.
22Incomplete ModelDid not explain where the electrons are located in the space surrounding the nucleus.