Atoms and Elements

1. Atoms and Elements Kim Shih Ph.D. Kim Shih

2. How many times we can cut this cake? Kim Shih

3. Scanning Tunneling Microscope • Gerd Bennig and Heinrich Rohrer found that as you pass a sharp metal tip over a flat metal surface, the amount of current that flows varies with distance between the tip and the surface Kim Shih

4. Scientists found that not only can you see the atoms on the surface, but the instrument allows you to move individual atoms across the surface Kim Shih

5. Law of Conservation of Mass • In a chemical reaction, matter is neither created nor destroyed • Total mass of the materials you have before the reaction must equal the total mass of the materials you have at the end Antoine Lavoisier 1743-1794 • total mass of reactants = total mass of products Kim Shih

6. Law of Conservation of Mass chemical formula 2HgO 2Hg + O2 chemical equation 2 Hg in reactant 2 O in reactant 2 Hg in product 2 O in product Kim Shih

7. Hg(NO3)2(aq) + 2KI(aq) HgI2(s) + 2KNO3(aq) Law of Conservation of Mass 3.25 g + 3.32 g = 6.57 g 4.55 g + 2.02 g = 6.57 g Kim Shih

8. Reaction of Sodium with Chlorine to Make Sodium Chloride • The mass of sodium and chlorine used is determined by the number of atoms that combine • Only whole atoms combine and atoms are not changed or destroyed in the process, the mass of sodium chloride made must equal the total mass of sodium and chlorine atoms that combine together 7.7 g Na + 11.9 g Cl2 19.6 g NaCl Kim Shih

9. Law of Definite Proportions Joseph Proust 1754-1826 • All samples of a given compound, regardless of their source or how they were prepared, have the same proportions of their constituent elements Kim Shih

10. Law of Definite Proportions 18g of H2O = 2g of H2 + 16g of O2 Law of Definite Proportions: Different samples of a pure chemical compound always contain the same proportion of elements by mass. By mass, water is: 88.8 % oxygen 11.2 % hydrogen Kim Shih

11. Proportions in Sodium Chloride A 100.0 g sample of sodium chloride contains 39.3 g of sodium and 60.7 g of chlorine A 200.0 g sample of sodium chloride contains 78.6 g of sodium and 121.4 g of chlorine A 58.44 g sample of sodium chloride contains 22.99 g of sodium and 35.44 g of chlorine Kim Shih

12. If a 10.0 g sample of calcite contains 4.0 g of calcium, how much calcite contains 0.24 g of calcium? 10 g X g = 4.0g 0.24g X= 0.60g Kim Shih

13. Law of Multiple Proportions John Dalton 1766-1844 • When two elements ( A and B) form two different compounds, the masses of B that combine with 1 g of A can be expressed as a ratio of small, whole numbers nitric oxide: nitrous oxide: 8 grams oxygen per 7 grams nitrogen 16 grams oxygen per 7 grams nitrogen Kim Shih

14. Law of Multiple Proportions Kim Shih

15. Oxides of Carbon • Carbon combines with oxygen to form two different compounds, carbon monoxide and carbon dioxide • Carbon monoxide contains 1.33 g of oxygen for every 1.00 g of carbon • Carbon dioxide contains 2.67 g of oxygen for every 1.00 g of carbon • Because there are twice as many oxygen atoms per carbon atom in carbon dioxide of in carbon monoxide, the oxygen mass ratio should be 2 Kim Shih

16. Hematite contains 2.327 g of Fe for every 1.00 g of oxygen. Wüsite contains 3.490 g of Fe per gram of oxygen. Show these results are consistent with the Law of Multiple Proportions. 1.5 = 3:2 Kim Shih

17. Path of Discovery of Atom and Electron Kim Shih

18. Dalton’s Atomic Theory • Each element is composed of tiny, indestructible particles called atoms • All atoms of a given element have the same mass and other properties that distinguish them from atoms of other elements • Atoms combine in simple, whole-number ratios to form molecules of compounds • In a chemical reaction, atoms of one element cannot change into atoms of another element • they simply rearrange the way they are attached Kim Shih

19. Copper atoms can combine with zinc atoms to make gold atoms • True • False Kim Shih

20. Water is composed of many identical molecules that have one oxygen atom and two hydrogen atoms • True • False Kim Shih

21. Some carbon atoms weigh more than other carbon atoms • True • False Kim Shih

22. The mass ratio of Fe:O in wüsite is 1.5 times larger than the Fe:O ratio in hematite, there must be 1.5 Fe atoms in a unit of wüsite and 1 Fe atom in a unit of hematite • True • False Kim Shih

23. Atomic Structure: Electrons Cathode-Ray Tubes: J. J. Thomson (1856–1940) proposed that cathode rays must consist of tiny, negatively charged particles which we now call electrons. Kim Shih

24. Thomson’s Experiment Investigate the effect of placing an electric field around tube 1. charged matter is attracted to an electric field 2. light’s path is not deflected by an electric field +++++++++++ Anode Cathode (+) (-) ------------- - + Power Supply Kim Shih

25. The charge/mass ratio of these particles: −1.76 x 108 C/g The charge/mass of the hydrogen ion: +9.58 x 104 C/g Kim Shih

26. Thomson’s Results • The cathode rays are made of tiny particles, particles were the ultimate building blocks of matter • These particles have a negative charge • because the beam always deflected toward the + plate • The amount of deflection was related to two factors, the charge and mass of the particles • Every material tested contained these same particles • The charge:mass ratio of these particles was −1.76 x 108 C/g • the charge/mass of the hydrogen ion is +9.58 x 104 C/g These cathode ray particles became known as electrons Kim Shih

27. Millikan’s Oil Drop Experiment Discovery: Oil droplet was always a whole number multiple of 1.6x 10-19 C. Kim Shih

28. Electrons • Electrons are tiny, negatively charged particles found in all atoms • Cathode rays are made of streams of electrons • The electron has a charge of −1.60 x 10-19 C • The electron has a mass of 9.1 x 10−28 g Kim Shih

29. Thomson’s Plum Pudding Atom • The structure of the atom contains many negatively charged electrons • These electrons are held in the atom by their attraction for a positively charged electric field within the atom • there had to be a source of positive charge because the atom is neutral • Thomson assumed there were no positively charged pieces outside because none showed up in the cathode ray experiment Kim Shih

30. Path of Discovery of Nucleus and Proton Kim Shih

31. Predictions of the Plum Pudding Atom • The mass of the atom is due to the mass of the electrons within it • electrons are the only particles in Plum Pudding atoms, therefore the only source of mass • The atom is mostly empty space • should not have a bunch of negatively charged particles near each other as they would repel Kim Shih

32. Rutherford’s Experiment • How can you prove something is empty space? • Put something through it! • use large target atoms • use very thin sheets of target so it will not absorb “bullet” • use very small particle as bullet with very high energy • but not so small that electrons will affect it • Bullet = alpha particles, target atoms = gold foil • a particles have a mass of 4 amu & charge of +2 c.u. • gold has a mass of 197 amu & is very malleable Kim Shih

33. Radioactivity • In the late 1800s, Henri Becquerel and Marie Curie discovered that certain elements would constantly emit small, energetic particles and rays • These energetic particles could penetrate matter • Ernest Rutherford discovered that there were three different kinds of emissions • alpha, a, rays made of particles with a mass 4x H atom and + charge • beta, b, rays made of particles with a mass ~1/2000th H atom and – charge • gamma, g, rays that are energy rays, not particles Marie Curie 1867-1934 Kim Shih

34. Kim Shih

35. Rutherford’s Results • Over 98% of the a particles went straight through • About 2% of the a particles went through but were deflected by large angles • About 0.005% of the a particles bounced off the gold foil Kim Shih

36. Rutherford’s Conclusions • Atom mostly empty space • because almost all the particles went straight through • Atom contains a dense particle that is small in volume compared to the atom but large in mass • because of the few particles that bounced back • This dense particle is positively charged • because of the large deflections of some of the particles Kim Shih

37. Plum Pudding Atom • • • • If atom was like a plum pudding, all the a particles should go straight through • • • • • • • • • • • • • • • • • • A few of the a particles do not go through Nuclear Atom . Almost all a particles go straight through . Some a particles go through, but are deflected due to +:+ repulsion from the nucleus . Kim Shih

38. Rutherford’s Interpretation –the Nuclear Model • The atom contains a tiny dense center called the nucleus • the amount of space taken by the nucleus is only about 1/10 trillionth the volume of the atom • The nucleus has essentially the entire mass of the atom • the electrons weigh so little they give practically no mass to the atom • The nucleus is positively charged • the amount of positive charge balances the negative charge of the electrons • The electrons are dispersed in the empty space of the atom surrounding the nucleus Kim Shih

39. Structure of the Nucleus • Nucleus had a particle that had thesame amount of charge as an electron but opposite sign – these particles are called protons • based on measurements of the nuclear charge of the elements • Protons are subatomic particles found in the nucleus with a charge = +1.60 x 10-19 C and a mass = 1.67262 x 10−24 g • Protons and electrons have the same amount of charge, for the atom to be neutral there must be equal numbers of protons and electrons Kim Shih

40. Path of Discovery of Neutron Kim Shih

41. Some Problems • How could beryllium have four protons stuck together in the nucleus? • shouldn’t they repel each other? • If a beryllium atom has four protons, then it should weigh 4 amu(atomic mass unit); but it actually weighs 9.01 amu! Where is the extra mass coming from? Kim Shih

42. Rutherford and Chadwick proposed that there was another particle in the nucleus – it is called a neutron • Neutrons are subatomic particles with a mass = 1.67493 x 10−24 g and no charge, and are found in the nucleus • 1 amu • slightly heavier than a proton • no charge Kim Shih

43. Atomic Structure The mass of the atom is primarily in the nucleus. The charge of the proton is opposite in sign but equal to that of the electron. Kim Shih

44. Elements • Each element has a unique number of protons in its nucleus • The number of protons in the nucleus of an atom is called the atomic number • the elements are arranged on the Periodic Table in order of their atomic numbers • Each element has a unique name and symbol • symbol either one or two letters • one capital letter or one capital letter and one lowercase letter Kim Shih

45. 14 12 C C 6 6 Atomic Numbers carbon-12 mass number 6 protons 6 electrons 6 neutrons atomic number carbon-14 mass number 6 protons 6 electrons 8 neutrons atomic number Kim Shih

46. 52 Cr 24 Question: How many protons, electrons, and neutrons are in an atom of ? Kim Shih

47. Isotopes • Soddy discovered that the same element could have atoms with different masses, which he calledisotopes • The observed mass is a weighted average of the weights of all the naturally occurring atoms Kim Shih

48. Isotopes • All isotopes of an element are chemically identical • undergo the exact same chemical reactions • All isotopes of an element have the same number of protons • Isotopes of an element have different numbers of neutrons • Isotopes of an element have different masses • Isotopes are identified by their mass numbers, which is the sum of all the protons and neutrons in the nucleus Kim Shih

49. Symbol Number of Protons Number of Neutrons A, Mass Number Percent Natural Abundance Ne-20 or 10 10 20 90.48% Ne-21 or 10 11 21 0.27% Ne-22 or 10 12 22 9.25% Neon Kim Shih

50. Isotopes Why is the atomic mass of the element carbon 12.01 amu? carbon-12: 98.89 % natural abundance 12 amu carbon-13: 1.11 % natural abundance 13.0034 amu mass of carbon = (12 amu)(0.9889) + (13.0034 amu)(0.0111) = 11.87 amu + 0.144 amu = 12.01 amu Kim Shih