# 5720 g 3437 g 1664 nbpcl3andpcl5are knowncompounds 5

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Unformatted text preview: 7.46/22.54) = 3.437 g Cl In compound B, we would expect 1 gram (85.12/14.88) = 5.720g Cl Dalton would predict that the ratio of masses of Cl is a “simple ratio of integers”. 5.720 g 3.437 g = 1.664 ≈ N.B.!PCl3!and!PCl5!are! known!compounds.! 5 3 Atomic Theory According to Dalton, compounds are made of units which he called atoms. These atoms behave according to a fixed set of rules. John Dalton Father of the Atomic Theory Basic postulates of Dalton’s atomic theory: " Matter is composed of indivisible particles called atoms " Atoms of a given element and are identical in all respects " Atoms from different elements differ in mass and chemical behavior " Compounds are composed of two or more atoms of different elements. Their combination results in molecules. " In a reaction, atoms can be rearranged, (re)combined but neither created nor destroyed. " The law of conservation of mass follows directly from Dalton’s ideas. " Similarly, the law of constant composition follows from Dalton’s idea that atoms are indivisible. " Dalton’s atomic theory enables us to set-up a scale of relative masses since we know the relative ratios of elements in compounds. Example: We know that in LiCl, Li is 16.4% and Cl is 83.6%. Since we know that the mass of Li relative to Cl must be the same as the mass % in LiCl then: mass of Cl atom 83.6% = ≈5 mass of Li atom 16.4% € N.B.!Mass!of!Li:!6.9! Mass!of!Cl:!35.5! Following this approach, we can establish a table of relative atomic masses for all elements. Definition: We define atomic mass as the ratio of the mass of a given atom to the mass of some reference atom*. The current standard is Carbon. N.B. Atomic masses are dimensionless. Molecular mass is simply the sum of the masses of the constituent elements in a specified molecule. H2SO4 = 98 *Not all atoms of a given element have the same mass (isotopes) (e.g. 1H, 2H, 3H).! John!Dalton’s!Atoms:! nitrogen! Numerous!mistakes;!both!in!masses,!and!in!which!are!atoms!vs.! compounds!(lime,!soda,!potash!are!all!compounds).! Dalton’s symbols for chemical elements. Some of these “elements” are no...
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## This document was uploaded on 11/06/2013.

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