Discovery of atomic structure and periodicity

1. Discovery of atomic structure and periodicity Dr. Orla Ni Dhubhghaill o.nidhubhghaill@ucc.ie

2. Early days - Philosophy • Aristotle (384 – • 322 B.C.) • Matter is continuous • “Nature abhors a • vacuum” • Democritus (460 – 371 B.C.) • Matter made up of atoms and space http://dbhs.wvusd.k12.ca.us/webdocs/AtomicStructure/ AtomicStructure.html

3. 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 Early Days - Philosophy • Solution – Pierre Gassendi (1592 – 1655) • Atoms are the smallest indivisible part of • matter • Created by God

4. 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

5. 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

6. ‘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, 1797 Reacting elements combine in a constant ratio in a given compound

7. 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

8. Dalton’s chemical symbols Adapted from Dickerson, Gray, Haight, Chemical Principles, 3rd edition, 1979