discovery of atomic structure and periodicity dr. orla ni dhubhghaill [email protected]
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Early days - Philosophy
• Democritus (460 – 371 B.C.)
• Matter made up of atoms and space
• Aristotle (384 – 322 B.C.)• Matter is continuous• “Nature abhors a vacuum”
http://dbhs.wvusd.k12.ca.us/webdocs/AtomicStructure/AtomicStructure.html
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Early Days - Philosophy
• 1417 – rediscovery of De Rerum Natura by Lucretius
• 14th/15th century – “Division of natural things has a limit”
• Aristotle’s teachings thought to be correct
• Atomic Theory = Godlessness
• 1624 (Paris) – death penalty for those opposed to Aristotle’s teachings
Solution – Pierre Gassendi (1592 – 1655)• Atoms are the smallest indivisible part of matter• Created by God
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John Dalton (1766-1844)"Having been in my progress so often misled by taking for granted the results of others, I have determined to write as little as possible but what I can attest by my own experience”
Belief in atoms based on experimental evidence
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Dalton’s postulates• All matter is made up of atoms• Atoms of the same element are identical, in
mass and chemical properties• When a compound is decomposed, the
recovered atoms are unchanged; they can re-combine to form the same or new compound.
• Atoms of different elements combine in simple whole number ratios to form compounds
•http://www.slcc.edu/schools/hum_sci/physics/whatis/biography/dalton.html•http://www.rsc.org/publishing/journals/DT/article.asp?doi=b307622a
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‘Laws’ governing reactions
Law of conservation of mass – Lavoisier, 1789
Total mass of products of chemical reaction is the same as total mass of reactants.
Law of constant composition – Proust, 1797Reacting elements combine in a constant ratio in a given compound
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Dalton’s Law of Multiple Proportions
• If two elements combine to form more than one compound, then the amounts of one element that combine with a fixed amount of the other will differ by factors that are the ratios of small whole numbers
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Dalton’s chemical symbols
Adapted from Dickerson, Gray, Haight, Chemical Principles, 3rd edition, 1979