To Understand The Atom Is To Understand Chemistry: Mileposts on the road to the atom

1. To Understand The Atom Is To Understand Chemistry: Mileposts on the road to the atom All matter is made from elements What makes elements unique? A component unique to each element A common component but assembled in a different way

2. 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 • Identify main features of the periodic table

3. Glimpse into the interior • What we “see” now with scanning tunneling microscope could only be speculated on 2,000 years ago

4. 400 BCProposal that matter is particulate • 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.

5. Democritus’ legacy • No indication about the size of these atoms. In fact Democritus atoms could be extremely large • Democritus’ idea has endured with some modifications: • Atoms are not uncuttable • Atoms are all very small • Atoms are themselves made of particles

6. Science in the dark ages • The decline of Greek civilization saw a concomitant decline of intellectual activity in Europe • The major “scientific” activity was alchemy, largely the pursuit of the transformation of matter into gold • Uncritical acceptance of Greek thinking about matter lingered until the Age of Enlightenment • 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 long-established conventions. Modern science is based on observation and reason

7. 1780Re-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

8. 1790Order out of chaosAntoine Lavoisier defines elements: substances that could not be broken down into simpler substances. • Beginnings of periodic table and definition of elements • Introduced concept of compounds (combination of elements) • Demonstrated Law of Conservation of Mass • No additional insights into the atom at this stage.

9. Chemical composition laws and arguments 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

10. 1803 - musing on atoms:John Dalton’s atomic theory • Sample of any element contains tiny particles called atoms • Atoms cannot be subdivided, created or destroyed • LawofConservationofMatter • 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

11. Significance of the Law of Multiple Proportions • With benefit of knowledge of chemical composition, consider example of carbon dioxide (CO2) and carbon monoxide (CO) • Combination of finite components (atoms) of C and O in simple number ratios is the most sensible explanation of the Law

12. 1808Gay-Lussac law of combining volumes: atomic implications • 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 belief that formula of water was HO (principle of simplicity)

13. Mystery of the gas volumes • Combining H and O in 1:1 ratio (Dalton’s proposed ratio) does not satisfy Conservation of Matter…

14. 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…?

15. Enter Avogadro:What if they are diatomic molecules? • Matter conserved, and each volume contains same number of particles

16. 1811Amadeo Avogadro’s hypothesis • Solved riddle of gas volumes by positing that the gas molecules contained two atoms. Most elemental gases are diatomic • Second part of hypothesis was that 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.

17. 1815William Prout weights of atoms appear to be simple multiples of the hydrogen atom. • Coincidence or significance? • This led to the proposal that atoms of heavier elements were made from hydrogen atoms • Implication that larger atoms comprise smaller units • Partial truth: there are common factors between atoms of different elements… • but they are not H atoms…

18. Development of atomic weight scale – elimination of the missing link • Experimentally mass could be measured • Without knowledge of atomic mass, impossible to know how many atoms of one element combine with another • Essential to know number of atoms to understand chemistry – write chemical formulae • Atomic weight scale, largely enabled by Avogardro, provides link between experimental observables and numbers of atoms

19. 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 atoms cannot be modified? What are the attractive forces? • What about those other experimental observations accumulating…?