Mileposts on the road to the atom

1. 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

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 Law of Multiple Proportions for atomic world • Illustration using carbon dioxide (CO2) and carbon monoxide (CO) • Combination of C and O as atoms in simple number ratios is most sensible explanation

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, 2:1 ratio (Gay Lussac’s expt) predicts only 1 vol of H2O, not 2 vols as observed. Hmmm...

15. Enter Avogadro:What if gases 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 molecules in the gas 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 are simple multiples of hydrogen atom. • Coincidence or significance? • Proposed 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. Atomic weight scale – the link between mass and number of atoms • Experimentally mass could be measured • Without knowledge of atomic mass, impossible to know how many atoms of one element combines with another • Essential to know number of atoms to understand chemistry • Atomic weight scale, enabled by Avogadro’s hypothesis, provides link between experimental observable (mass) and numbers of atoms

19. 1869 - elements orderedDimitri Mendeleev and the periodic table • The most important document in chemistry • Ordered the elements according to their atomic weights and properties. • Only 60 elements identified at the time • Predicted existence of undiscovered elements and their properties which were often proved startlingly accurate.

20. Facts of periodic tableThere are 91 naturally occurring elements: 2 liquids, 11 gases, 23-25 nonmetals

21. The Periodic Table: Groups and Periods Groups: columns of elements Periods: rows of elements

22. Group (family) behaviour • 1A ALKALI METALS: Reactive. Reactivity increases down group. Physical properties vary in a gradual fashion. • 2A ALKALINE EARTH METALS : Similar to alkalis but less reactive. Lots of common calcium compounds: calcium carbonate (limestone, marble), calcium oxide (lime), calcium sulphate (gypsum, plaster of Paris)

23. Important groups on nonmetal side • 7A HALOGENS: Very reactive lots of compounds with most elements. Chlorides in particular are common: NaCl. KCl. Reactivity decreases down group • 8A RARE, INERT, NOBLE GASES: All gases, discovered late because of lack of compounds. Not rare - 1 % argon in air; helium is second most abundant element in universe. Very unreactive.

24. Unanswered questions • What features of atoms are responsible for differences in element properties? • How can atoms actually bind together in compounds; what are the attractive forces? • What about those other experimental observations accumulating…?