The History of the Atom

1. Chapter 3 The History of the Atom

2. look at sugar; made of crystals grind them into finer powder, still sugar dissolved in water, they seem to disappear look underneath microscope, can’t see them, but still can taste them these observations led many philosophers to ponder the fundamental nature of matter The atom: philosophical idea to scientific theory

3. 400 BC Greek philosopher Democritus believed that matter was made of smaller particles he called ATOMS • atomos- Greek meaning indivisible

4. Aristotle did not believe in atoms • believed that matter was continuous and made of 4 elements: fire, water, air, and earth • due to his popularity, his theory was accepted for 2000 years

5. Alchemists were the next group to impact the atomic theory • main goal: find the smallest particle of matter through which they could change impure metals into gold & the fountain of youth • left a legacy of lab techniques still in use today

7. by late1700s, all chemists accepted modern definition of element-subst that can’t be further broken down by ordinary chemical means also accepted that elements form compounds that have properties different from the elements that make them up new balances brought about quantitative analysis of compounds

8. several laws were discovered: law of conservation of mass-mass is neither created nor destroyed during ordinary chemrxns or physical changes

9. + = Carbon, C Oxygen, O Carbon Monoxide, CO Mass x Mass y Mass x + Mass y + Carbon, C Oxygen, O Carbon Monoxide, CO Mass x Mass y Mass x + Mass y Law of Conservation of Mass = Pg 65-69 Sec 3-1

10. law of definite proportions- a chem compound contains the same elements in exactly the same proportions by mass regardless of the size of the sample or source of compound

11. + 1 Carbon 2 Oxygen Law of Definite Proportions ALWAYS + = 1 Carbon 1 Oxygen Carbon Monoxide ALWAYS = Carbon Dioxide, CO2 Pg 65-69 Sec 3-1

12. also known at this time: 2 elements sometimes will combine to form more than one compound law of multiple proportions- if 2 or more different compounds are composed of the same 2 elements, then the ratio of the masses of the 2nd element combined with a certain mass of the 1st element is always a ratio of small whole numbers

13. + Carbon Oxygen Law of Multiple Proportions + = Carbon Oxygen Carbon Monoxide, 1:1 1 1 = Carbon Dioxide, 1:2 1 2 Pg 65-69 Sec 3-1

14. in 1808, John Dalton, an English schoolteacher, proposed explanation for laws: elements are composed of atoms that are going to combine in whole numbers led to Dalton’s Atomic Theory john dalton

16. ELEMENT 1 ELEMENT 2 ELEMENT 3 ELEMENT 4 Pg 65-69 Sec 3-1

17. + + Pg 65-69 Sec 3-1

18. Democritus’s idea was turned into scientific theory that could be tested by experiments not all Dalton’s theory have been proven to be true, but through the testing of other scientists over time the picture of the modern atom has evolved

19. exceptions to Dalton’s theory: atoms are divisible • atoms consist of 2 regions: • nucleus- very small region in center of atom • consists of neutrons (particles with no electric charge) and protons (particles with positive charge) • electron cloud- large region surrounding nucleus • consists of electrons (particles with negative charge) atomic structure

20. J.J.Thomson

21. 1897 closed glass tube w/ low pressure gas inside positive anode on one end; negative cathode on other pass electricity from one end to other rays moved from cathode to anode Thompson suggested these rays were streams of negative particles called electrons, e-

22. rays were deflected by negatively charged plates • studied the degree of deflection of rays and was able to determine the charge to mass ratio

23. Robert Millikan • in 1909, using apparatus, he determined charge on e- • used atomizer to spray fine droplets of oil • e- attached to oil droplets which fell through hole into vacuum chamber

24. gravity caused droplets to fall down • by adjusting the electric charge on the plates in chamber, kept oil droplets suspended • calculated charges on droplets

26. using Thomson’s data and his own, he was able to calculate the mass of the e- mass is 9.09 x 10-28 g e- mass is so minute that it is assigned a mass value of 0 e- has a charge of -1

27. in 1886, a stream of positively charged particles was observed moving from anode (+) to cathode (-) called canal rays deflected by positive electric fields these were named protons, p+ determined that the electric charge was opposite that of e- Eugen Goldstein

28. mass of p+ is 2000x that of e- protons have an assigned mass of 1

29. famous Gold Foil experiment in 1910 bombarded gold foil w/ alpha particles (+) from radioactive source fluorescent screen around foil would determine how alpha particles scatter markings on screen were counted to determine degree of deflection Ernest Rutherford, et al

31. most particles went through foil with little or no deflection few reflected at large angles and some almost back toward source analyzed mathematically

32. discovered: atoms mostly empty space positive core in center of atom core contains mass of atom named core nucleus

33. rutherford, bohr, and geiger

34. Henry Moseley • in 1913, determined magnitude of and charge in elements • studied X-rays produced in X-ray tubes w/ anodes of diff metals

35. results indicated that each element differed from preceding one by having 1 more positive charge in nucleus • charge was always same for a given element

36. X-ray spectra indicated # of protons led to the atomic number atomic number is the number of protons in nucleus of atom element’s identity is due to number of protons in it

37. JJ Thomson conducted exp w/ neon gas atoms observed that there were 2 diff types of neon atoms; alike chemically, but diff in mass named isotopes- atoms of the same element that differ in mass; have the same # of p, but varies in number of neutrons

38. in 1932, Chadwick bombarded Be w/ high energy alpha particles this produced some uncharged particles w/ mass ~equal to that of p+ named neutrons, n0 neutrons assigned mass of 1 jameschadwick

39. 1896, discovered uranium ore exposed sealed photographic film Pierre & Marie Curie discovered rays were given off by U & Ra these rays had an effect on charged electroscope discovered radioactive substance henribecquerel

40. radioactivity- phenomenon of rays being produced spontaneously by unstable atomic nuclei rays can be particles or energy or mixture of both

41. becquerel and the curies