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Atoms: discovering the stuff that stuff is made of We know all matter is made from elements What makes each element unique? A component unique to each element? A common component but assembled in a different way? Learning Objectives Describe what is meant by “atom” Describe Law of Conservation of Mass and Definite Proportions Identify main features of Dalton’s atomic theory Describe Avogadro’s Hypothesis Glimpse into the interior What we “see” now with scanning tunneling microscope could only be speculated on 2,000 years ago The scale of things From a distance sand looks smooth and continuous Up close it’s made of particles Each particle looks smooth Up closer it is made of particles - atoms 400 BC Beginnings of the atom Democritus posed the question: could matter be subdivided forever? He answered no: there is a limit to the extent to which matter can be subdivided, and he coined the term atom from the Greek for uncuttable a-tomos. Democritus’ idea is not much different from the modern atom with some modifications... Atoms are not uncuttable Atoms are all very small Atoms are themselves made of particles Stop forward movement: Science in the dark ages The decline of Greek civilization saw concomitant decline of intellectual activity in Europe Science and maths continued in Persian Empire (golden age) The major “scientific” activity was alchemy, largely the pursuit of the transformation of matter into gold and the elixir of life Uncritical acceptance of Greek thinking about matter lingered until the Age of Enlightenmen Matter is made of the four elements (earth, wind and fire...and water) The modern scientific era began in the 17th century, pioneered by a few inspired individuals who broke free from longestablished conventions Models for nature were based on observation and experiment 1780 Re-emergence of the atom: Daniel Bernoulli’s kinetic theory of gases Atoms were assumed to be hard round spheres which behaved much like billiard balls. Success of kinetic theory in describing gases lends support to atomic description of matter. It is still used, with scarcely any modification 1790 Order out of chaos Antoine Lavoisier and the elements Beginning of the periodic table and the concept of elements Introduced concept of compounds – elements combined Demonstrated Law of Conservation of Mass No additional insights into the atom at this stage. Chemical laws and the case for an atomic world Law of Conservation of Mass: Matter is neither created nor destroyed in the course of a chemical reaction. Does not apply to nuclear changes Law of Definite Proportions: In forming compounds, elements combine together in definite mass ratios No knowledge of actual atom ratios at this stage, but how else to explain fixed quantities unless the elements were present as discrete bodies Strong indication for combination of the atoms in simple ratios 1803 John Dalton’s atomic musings Sample of any element contains tiny particles called atoms Atoms cannot be subdivided, created or destroyed Law of Conservation of Matter All atoms of the same element are the same All atoms of different elements are different Atoms combine together in simple whole number ratios Law of Multiple Proportions: The ratio of the masses of one element combined with the same mass of another element is a simple whole number Significance of the Law of Multiple Proportions With benefit of knowledge of chemical composition, consider example of carbon dioxide (CO2) and carbon monoxide (CO) MassOCO2 MassOCO 2 Combination of finite components (atoms) of C and O in simple number ratios is the most sensible explanation of the Law 1808 Gay-Lussac law of combining volumes Gases react with other gases to give products, in volumes which have simple whole number ratios. “Mystery” of the reaction of hydrogen and oxygen to provide water. 2 vols of hydrogen + 1 vol of oxygen gives 2 vols of water (Why not 1 vol?) The ratio of 2:1 was inconsistent with Dalton’s unproven belief that the formula of water was HO Mystery of the gas volumes Combining H and O in 1:1 ratio (Dalton’s proposed ratio) does not satisfy Conservation of Matter… Needs another box of O atoms H:O = 2:1 However, the 2:1 ratio (as suggested by volume ratio of reactants) predicts only 1 vol of H2O, not 2 vols as observed…? Enter Avogadro: What if they are diatomic molecules? Matter conserved, and each volume contains same number of particles 1811 Amadeo Avogadro’s hypothesis Solved riddle of gas volumes by positing that the molecules in the gas contained two atoms. Most elemental gases are diatomic The same volumes of all gases contain the same number of particles. Results conflicted with Dalton’s views and were not recognized for nearly 50 years Key result: Able to calculate the relative atomic masses of the elements: development of a scale of atomic weight. 1815 William Prout: weights of atoms appear to be simple multiples of hydrogen Coincidence or significance? Proposed that atoms of heavier elements are made of hydrogen atoms Implies that larger atoms comprise smaller units Partial truth: there are common factors between atoms of different elements… but not H atoms… Unanswered questions What features of the atoms are responsible for the differences in element properties? How can atoms actually bind together in compounds to give substances of different properties if the atoms cannot be modified what would the attractive forces be? What about those other experimental observations accumulating…?