Atomic History

1. Atomic History

2. Ancient History Democritus Greece • , who was from was the first person to use the term atom during B.C. • Democritus defined the atom as being . • opposed Democritus because believed that atoms and that matter was . 400 indivisible Aristotle Aristotle did not exist continuous

3. Foundations of Atomic Theory • Chemists accepted the modern definition of an element during the late . • An is a substance that cannot be broken down by ordinary chemical means. • Chemists could not agree whether elements always combine in the when forming a compound. 1700s element same ratio

4. Foundations of Atomic Theory chemical reactions • Chemists studied during the 1790s in order to find out whether elements always combine in the same ratio. • They discovered the law of , which states that conservation of m ass

5. Foundations of Atomic Theory chemical reactions • Chemists studied during the 1790s in order to find out whether elements always combine in the same ratio. • They discovered the law of , which states that cannot be or during chemical and physical changes. conservation mass of m ass created destroyed

6. High voltage High voltage After reaction O2 H2O 0 g H2 + 40 g O2 85 g total Conservation of Mass electrodes Before reaction glass chamber 5.0 g H2 H2 O2 + 80 g O2 45 g H2O 85 g total Dorin, Demmin, Gabel, Chemistry The Study of Matter , 3rd Edition, 1990, page 204

7. Law of Definite Proportions same • The law of definite proportions deals with the compound. • The fact that a compound consists of the percentages by regardless of of each element you start with, the compound is formed, or the compound is formed. always same mass how much where how

8. copper carbonate 51.44% copper 38.85% oxygen 9.72% carbon Example of the law of definite proportions • In the laboratory… • In nature… (copper ore in Nevada) + + 51.44% Cu 38.85% O 9.72% C

9. Law of Multiple Proportions • The law of multiple proportions states: when two of the elements combine to form compounds and the first element has a mass, the of masses between the element will be a number. same different ratio fixed second element whole

10. Example of the law of multiple proportions • Two compounds containing carbon and oxygen Carbon dioxide (CO2) Carbon monoxide (CO) 1.0g C 1.33g O 1.0g C 2.66g O

11. Dalton’s Atomic Theory atoms same • All matter is made up of . • Atoms of the element are and atoms of elements are . • Atoms cannot be ,, or . • Atoms of different elements combine in to form compounds. • In chemical reactions, atoms are , or . identical different different subdivided created destroyed whole-number ratios combined separated rearranged

12. Modern Atomic Theory atoms same • All matter is made up of . • Atoms of the element are and atoms of elements are . • Atoms cannot be ,, or . • Atoms of different elements combine in to form compounds. • In chemical reactions, atoms are , or . 2. Atoms of different elements are different identical different different subdivided created destroyed whole-number ratios combined separated rearranged

13. Cathode Ray Experiment Joseph John (J.J.) Thomson • conducted the cathode ray experiment in . 1897

14. Cathode Ray Experiment Cathode ray tubes • are tubes that are filled with low pressure gas and contain two metal disks. - + Cathode ray tube metal disks

15. Cathode Ray Experiment voltage source - + vacuum tube metal disks

16. Cathode Ray Experiment voltage source ON - OFF + Passing an electric current through a cathode ray tube causes the tube to . glow

17. Modern Atomic Theory stream particles • The glow was caused by a of called a cathode ray. • A cathode ray is by a magnetic field. deflected

18. Findings from the Cathode Ray Experiment charge:mass • Thomson measured the ratio of the cathode ray particles and found that this ratio was ALWAYS the for every element. • Thomson concluded that cathode rays are composed of identical . charged . same negatively particles

19. Charge and Mass of the Electron Robert A. Millikan 1909 • studied electrons in . • Millikan found that the of an electron is of the mass of a atom. • He confirmed that an electron carries a charge. • Millikan concluded that are present in atoms of elements. mass 1/2000 hydrogen negative electrons all

20. Charge and Mass of the Electron • The discovery of the electron led chemists to infer the following during the early 1900s: • Atoms must contain a charge in order to the because atoms are electrically . • Atoms must contain other that account for most of their because of the tiny mass of the electron positive balance electrons neutral particles mass

21. The Gold Foil Experiment Rutherford and associates 1911 • conducted the gold foil experiment in . • are positively charged particles. • Rutherford and associates shot a thin piece of with fast-moving . • They assumed that and were distributed throughout the gold foil. Alpha particles gold foil alpha particles mass charge uniformly

22. What he expected… He expected that the alpha particles would the gold foil pass through

23. What he got… majority He found that the of the alpha particles passed through while other alpha particles were redirected

24. Findings from the Gold Foil Experiment • Rutherford explained that the alpha particles were redirected because they experienced a , which occupies a very of space. • He concluded that the force was due to a bundle of matter with a charge, which he called the . powerful force small amount dense positive nucleus

25. Other discoveries Goldstein 1886 • observed cathode ray tubes in and found rays that traveled in the direction of cathode rays • He concluded that these oppositely traveling rays contained charged particles. • discovered the existence of the in . opposite positively Chadwick neutron 1932

26. Composition of the Atomic Nucleus

27. Composition of the Atomic Nucleus

28. Composition of the Atomic Nucleus

29. Composition of the Atomic Nucleus

30. Composition of the Atomic Nucleus

31. Composition of the Atomic Nucleus

32. Composition of the Atomic Nucleus

33. Composition of the Atomic Nucleus • Atoms are electrically neutral because they contain the number of and . • The nuclei of atoms of different elements differ in the . same protons electrons number of protons

34. Forces in the Nucleus • Atoms with two or more protons do not fall apart because there is a strong between them. • The are the short-range forces between protons and protons, neutrons and neutrons, and protons and neutrons that hold the nucleus together. attraction nuclear forces

35. Forces in the Nucleus electron cloud • The is a region occupied the electrons. • Atomic radii are expressed in and they range from to in length. picometers (pm) 40pm 270pm

36. Exit Ticket • What are the two most important ideas of Dalton’s atomic theory? • Which of the following did Thomson NOT discover about the cathode ray? • The charge to mass ratio of a cathode ray is large • The mass of a cathode ray is 1/2000 the mass of a hydrogen atom • Cathode rays are made up of negatively charged particles • What did Rutherford NOT discover about an atom? • The nucleus contains neutrons • The nucleus contains positively charged particles • The volume of a nucleus is very tiny