Ch. 3: Atoms

1. Ch. 3: Atoms Sec. 1: From Philosophical Idea to Scientific Theory

2. History of the atom • Greeks were first to try to explain chemical reactions • 400 BC: thought all matter composed of: • Fire • Earth • Water • Air • Democritus first used word “atomos”, meaning indivisible Nothing exists but atoms and empty space; everything else is opinion. (Demokritos)

3. Next 2000 years • Alchemy gave basis for modern chemistry • Two ideas • Turning cheap metals into gold and silver • Finding the “elixir of life” • Did identify elements and learn how to prepare acids • Very secretive

4. Robert Boyle (1627-1691) • Defined element: any substance that cannot be broken down into simpler substances • Felt science supported religion • End of four element theory

5. Robert Boyle (1627-1691) (Not necessary to copy what is in italics) “I now mean by elements, as these chymists that speak plainest do by their principles, certain primitive and simple or perfectly unmingled bodies, which not being made of any other bodies, or of one another, are the ingredients of which all those called perfectly mixt bodies are immediately compounded, and into which they are ultimately resolved.”

6. Antoine Lavoisier • Father of chemistry • Lost his head during the French Revolution • Devised metric system in 1771 • Responsible for naming and listing elements of the day

7. Lavoisier’s wife played a big role in his work, unusual for the times.

8. The Elements • 110 known elements • 88 (?) occur naturally The elements form a plethora of compounds, just as 26 letters of the alphabet make a seemingly endless number of words.

9. Elements in Earth’s Crust, Oceans and Atmosphere* • These eight elements account for approximately 98.5% of the total mass of the earth's crust. • oxygen (46.6%) calcium (3.6%) • silicon (27.7%) sodium (2.8%) • aluminum (8.1%) potassium (2.6%) • iron (5.0%) magnesium (2.1%) *Know the top two.

10. Elements in the Human Body* • Oxygen (65%) • Carbon (18%) • Hydrogen (10%) • Nitrogen (3%) • Calcium (1.5%) • Phosphorus (1.0%) • Potassium (0.35%) • Sulfur (0.25%) • Sodium (0.15%) • Magnesium (0.05%) • Copper, Zinc, Selenium, Molybdenum, Fluorine, Chlorine, Iodine, Manganese, Cobalt, Iron (0.70%) • Lithium, Strontium, Aluminum, Silicon, Lead, Vanadium, Arsenic, Bromine (trace amounts) *Know the top four.

11. Symbols for the Elements • Sources of element names come from the Greek, Latin & German languages • Symbols simplify writing by using abbreviations • 1st letter always capitalized (C, F) • 2nd letter always small case (Zn, Na)

12. Symbols from Latin Names Element Symbol Latin name Copper Cu cuprum Gold Au aurum Lead Pb plumbum Mercury Hg hydrargyrum Potassium K kalium Silver Ag argentum Sodium Na natrium Tin Sn stannum

13. Symbols and Compounds • Chemical formulas represent compounds • Symbols indicate the elements present • Subscripts show the relative numbers of atoms of each element

14. KCl: potassium and chlorine, 1 of each • CCl4: One atom of carbon and 4 atoms of chlorine • C6H12O6

15. Atomic Theory States that all matter is composed of discrete units called atoms

16. Democritus • Matter consists of basic particles that cannot be divided (atoms) • “Atomos” means uncut or indivisible

17. 18th century Observations • Natural materials are mixtures of pure substances • Pure substances can be either elements or compounds (Water was thought to be an element until electrical current was used to separate the hydrogen and oxygen

18. Law of Conservation of Mass Mass is neither created nor destroyed in a chemical or physical reaction

19. 18th century Observations, cont. • Law of Constant Composition (Proust) • A given compound always has the same composition, regardless of where it comes from • Law of Definite Proportions • When elements combine to form chemical compounds, they unite in definite proportions by mass

20. In 18 grams of water, there are 2 g of hydrogen and 16 g of oxygen • In 72 g of water, there are 8 g of hydrogen and 64 g of oxygen • Ratio of hydrogen to oxygen is always 1:8

21. John Dalton (1766–1844) • Englishman • Was a school teacher at age 12 • Extremely interested in meteorology • Collected air samples all over Europe • Made daily weather observations for 57 years (Jeff Lyons would have loved him!) • Wondered about the composition of the air, which led to his atomic theory

22. Dalton’s Atomic Theory (1808-1810) • All elements are made of tiny particles called atoms • All atoms of an element are identical • All atoms of a given element are different from those of any other element

23. Atoms of 1 element can combine with atoms of other elements to form compounds. 5.Atoms are indivisible, and are neither created or destroyed. Reactions simply change the way atoms are grouped together.

24. A given compound contains the same relative numbers and types of atoms

25. We might as well attempt to introduce a new planet into the solar system, or to annihilate one already in existence, as to create or destroy a particle of hydrogen. John Dalton, A New System of Chemical Philosophy, 1808)

26. Dalton’s Symbols for the Elements

27. John Dalton was the first person to use ball-and-stick models to represent molecules. He asked his friend Peter Ewart, a Manchester engineer, to make a series of them for him in about 1810. These wooden balls were made to demonstrate Dalton's theory of how atoms in solids, liquids and gases are arranged. He used these models in his lectures and for other demonstrations, but failed to publish details of them until 1842, two years before his death.

31. 12 g C 16g O 12 g C 32 g O For the same amount of carbon, the ratio of oxygen in the 2 compounds is 1:2.

32. Dalton's Model • Not accepted at first • Law of Multiple Proportions • Relates to two compounds made by the same elements • Carbon dioxide has twice the mass of oxygen as carbon monoxide

33. Changes to Dalton’s Theory • Took until early 1900’s • Atom was divisible (nuclear physics) • Isotopes existed • We still use his model (with some revisions) today

34. Modern Atomic Theory Changes to Dalton’s theory: • Atoms of an element are nearly identical, but may have different masses • Atoms are not indivisible, but are composed of subatomic particles • Matter is neither created nor destroyed in ordinary chemical reactions

35. Pop Quiz • What are the 2 most abundant elements in the human body? • Write 3 postulates of Dalton’s atomic theory. • Describe the Law of Multiple Proportions

36. Pop Quiz • Oxygen and carbon • Elements are made of tiny particles called atoms • All atoms of an element are identical • All atoms of a given element are different from those of any other element • Atoms of 1 element can combine with atoms of other elements to form compounds. • Atoms are indivisible, and are neither created or destroyed. • Reactions simply change the way atoms are grouped together. • A given compound contains the same relative numbers and types of atoms

37. Sec. 2: The Structure of the Atom • J. J. Thomson (1897) • Discovered the electron • Plum Pudding Model • Ernest Rutherford • Showed existence of a nucleus

39. How J. J. Thomson used properties of cathode rays to hypothesize properties of the electron. observation ray properties are independent of the cathode material hypothesis cathode ray stuff is a component of all materials

40. How J. J. Thomson used properties of cathode rays to hypothesize properties of the electron. observation cathode rays bend near magnets hypothesis magnets bend the paths of moving charged particles; maybe cathode rays are streams of moving charged particles

41. How J. J. Thomson used properties of cathode rays to hypothesize properties of the electron. observation rays bend towards a positively charged plate.rays impart a negative charge to objects they strike. hypothesis cathode rays are streams of negative charges

42. How J. J. Thomson used properties of cathode rays to hypothesize properties of the electron. observation Cathode rays don't bend around small obstacles,cast sharp shadows,can turn paddlewheels placed in their path, and travel in straight lines hypothesis cathode rays behave like streams of particles (subatomic particles means smaller than atoms)

43. Gas at low pressure Positive field Cathode (-) Gas at low pressure Cathode Ray Electron Beam Paddle Wheel Cathode (-) Anode (+) Anode (+) Cathode Ray Electron Beam Negative field Metal Disk Paddle Wheel Track Electric Source

44. Thomson’s Conclusions • Cathode rays are negatively charged subatomic particles • Later named electrons • Also determine the ratio of the charge of electron to its mass to be approx. 1 x 107 (BIG number!)

45. Plum Pudding Model • Atom has a spherical cloud of positive charge (pudding) with electrons (plums) scattered within it that make the atom neutral

47. Robert Millikan • Discovered the charge of an electron • From that information and the charge/mass ratio, the mass of an electron was calculated to be about 1/2000 the mass of a hydrogen atom

48. Ernest Rutherford's scattering experiment hypothesis: • If the plum pudding model of the atom is correct, atoms have no concentration of mass or charge (atoms are 'soft' targets)

49. Experiment to test hypothesis: • fire massive alpha particles at the atoms in thin metal foil • alpha particles should pass like bullets straight through soft plum pudding atoms

50. observation: a few alpha particles ricocheted! http://micro.magnet.fsu.edu/electromag/java/rutherford/ http://www.mhhe.com/physsci/chemistry/essentialchemistry/flash/ruther14.swf