Atomic Theory and Nuclear Chemistry

1. Atomic Theory and NuclearChemistry

2. Early Models of the Atom • Democritus:c. 470-400 BC Greek Philosopher • matter is composed of tiny, discrete, indivisible particles called atomos(Greek word meaning indivisible). • Ideas based on philosophical speculation • Theory not accepted due to influence of Aristotle • An atom is the smallest part of an elementthat retains the chemical properties of that element. It cannot be broken down by ordinary means.

3. Alchemy in the Middle Ages • Matter composed of 4 elements • earth, air, fire and water. • Believed that any substance could be formed: precious metals to elixirs to cure disease and prolong life • Believed in transmutation • “turning lead into gold” cliché • Condemned by the Catholic church • hidden practices

4. Contributions of Alchemists • Produced hydrochloric acid, nitric acid, potash and sodium carbonate. • Able to identify the elements arsenic, antimony, and bismuth. • Invented and developed laboratory devices and procedures. • Laid the foundation for the development of chemistry as a scientific discipline.

5. Antoine Laurent Lavoisier • Law of Conservation of Matter states that matter is neither created nor destroyed, it only changes form. • 1st to announce that air was made up of 2 gases – oxygen and azote (nitrogen) • Work done on combustion, oxidation, and gases • Lavoisier is known as the Fatherof Chemistry.

6. In 1771, at age 28, Lavoisier married the 13-year-old Marie-Anne Pierrette Paulze. Over time, she proved to be a scientific colleague to her husband. She translated documents and chemistry books from English. She created many sketches and carved engravings of the laboratory instruments he used. She also edited and published Lavoisier’s memoirs after his death. She hosted parties at which eminent scientists discussed ideas and problems related to chemistry.

7. Engravings of Lavoisier’s Equipment by his wife

8. An appeal to spare his life so that he could continue his experiments was cut short by a judge saying: "The Republic needs neither scientists nor chemists; the course of justice cannot be delayed.” One and a half years following his death, Lavoisier was exonerated by the French government. When his private belongings were delivered to his widow, a brief note was included reading "To the widow of Lavoisier, who was falsely convicted." Lavoisier was Guillotined May 8, 1794

9. Joseph LouisProust: 1799 • Law of Definite Proportion states that compounds always have the same elements in the same proportion by mass. Ex) the ratio of H:O in water is always 2:16.

10. John Dalton:1766-1844 • English schoolteacher • Some of the original chemical symbols from his book:

11. John Dalton: 1803-1808 Proposed Atomic Theory of Matter: • An element is composed of extremely small, indivisible particles called atoms • All atoms of a given element have identical properties that differ from those of other elements • Atoms cannot be created, destroyed, or transformed into atoms of other elements

12. Dalton’s Atomic Theory (cont.) • Compounds are formed when atoms of different elements combine with one another in small whole-number ratios • In chemical reactions, atoms are combined, separated, or rearranged Dalton is credited as being the Father of the Modern Atomic Theory

13. Dalton (Billiard Ball) Modelatoms are solid, hard, indivisible spheres These wooden balls, were the first models made to represent atoms and were used by John Dalton (1766-1844) to demonstrate atomic theory. Credit: Science Museum/Science & Society Picture Library

14. Law of Multiple Proportionsproposed by Dalton • If 2 or more different compounds are composed of the same two elements, then the ratios of the masses of the 2nd element is always a ratio of small whole numbers • CO (1.0 g C/1.33 g O) • CO2 (1.0 g C/2.66g O) • 2:1 ratio of O in the compounds • NO (1.0 g N/1.14 g O) • NO2 (1.0 g N/2.28 g O) • 2:1 ratio of O in the compounds

15. Benjamin Franklin: 1706-1790American statesman/scientist Ben’s lightning rod in the Franklin Institute

16. In 1752 Benjamin Franklin • Experimented with electricity • He found that an object can have a positiveor a negative charge. negative and negative: repel negative and positive: attract positive and positive: repel

17. Michael Faraday (1839)English scientist • Hypothesized that atoms contain electric charge. • Built 1st electrical motor • Introduced words such as… • Ion, electrode, anode and cathode • A unit of electricity was named after him = farad • Static Electricity = electrons move and then are at rest (grounded)

18. William Crookes – 1875English scientist • Cathode Ray Tube: An evacuated glass tube with gas at low pressure • Electricity is passed through 2 electrodes: cathode (negative) and anode (positive) • Light is cast from cathode to anode (look at the shadow) • Magnet deflects light – this proved that particles have charge and mass.

19. Crookes’ Conclusion • Light is composed of negatively charged particles • Discovered based upon magnet deflection and anode shadow Crooke’s Maltese Cross You Tube Demo (CRT) Applying a Magnet Tutor Vista animation

20. Wilhelm Roentgen – 1895German scientist • In a Crookes tube at very low gas pressure, rays of unknown origin were discovered • Discovery called X rays • Glass fluoresced and the air around the equipment was ionized Wilhelm Roentgen’s X ray image of his wife’s hand

21. J.J. Thompson: 1897 • English Physicist who said a cathode ray is made of electrons, they have mass(9.1 x 10-28g)and are negatively charged particles. Thus he is credited with “discovering” electrons.

22. Cathode Rays and Electrons • Cathode Ray Tube: An evacuated glass tube where a beam of electrons flows from the cathode (negative electrode) to the anode (positive electrode.)

23. J.J. Thomson • Used Crookes tube (gas discharge tube) • Applied positive and negative field to a beam of cathode rays. The deflection was the same for all gases. • Experimentally proved the existence of the electron (e-) Discovery of the Electron 11 min Cathode Ray Tube (McGraw Hill) http://www.aip.org/history/electron/jjappara.htm

24. Thomson Experimented with hydrogen gas at low pressure • 2nd beam of particles was moving towards the cathode, therefore, positive particles • Deflection of positive ions varied with different gases • Hydrogen ions had the greatest deflection, therefore, the smallest positive mass • Hydrogen ion deflection was smaller than that of the electron, therefore more massive than an electron • Hydrogen ion = proton

25. J.J. Thomson • Calculated the charge to mass ratio using different cathode metals and different gases (e/m= 1.76x108 C/g) • Measured how much they were deflected by a magnetic field and how much energy they carried. • He found that the charge to mass ratio was over a thousand times higher than that of a hydrogen ion, suggesting either that the particles were very light or very highly charged. Credit:Science Museum/Science & Society Picture Library

26. J.J. Thomson • Made a bold conclusion: • Cathode rays were indeed made of particles which he called “corpuscles," and these corpuscles came from within the atoms of the electrodes themselves, meaning the atoms were, in fact, divisible. • Won a Nobel Prize in Physics in 1906.

27. J.J. Thomson: 1897 • Thought the atom was made up of these corpuscles (negative charges) distributed in a sea of positive charge • Related it to “plumpudding” Different models of the plum pudding model

28. Robert Millikan:1909American scientist • Oil drop experiment • Measured voltage to determine the charge on one electron = -1.60 x 10-19 coulomb/e- • Used Thomson’s charge to mass ratio to calculated the mass of an electron Mass of 1 electron = 9.11 x 10-28g Millikan Experiment 12 min

29. An atomizer sprayed a fine mist of oil droplets into the upper chamber. Some of these tiny droplets fell through a hole in the upper floor into the lower chamber of the apparatus. • Next, Millikan applied a charge to the falling drops by irradiating the bottom chamber with x-rays. This caused the air to become ionized - meaning the air particles lost electrons. • A part of the oil droplets captured one or more of those extra electrons and became negatively charged • By attaching a battery to the plates of the lower chamber he created an electric field between the plates that would act on the charged oil drops • He adjusted the voltage till the electric field force would just balance the force of gravity on a drop, and the drop would hang suspended in mid-air. • Some drops have captured more electrons than others, so they will require a higher electrical field to stop • Particles that did not capture any of that extra electrons were not affected by the electrical field and fell to the bottom plate due to gravity. • When a drop is suspended, its weight  m · g  is exactly equal to the electric force applied, the product of the electric field and the charge q · E. • The values of E (the applied electric field), m (the mass of a drop which was already calculated by Millikan), and g (the acceleration due to gravity), are all known values. Unknown charge on the drop, q m · g = q · E • Millikan repeated the experiment numerous times varying the strength of the x-rays ionizing the air so that differing numbers of electrons would jump onto the oil molecules each time. • He obtained various values for q. The charge q on a drop was always a multiple of 1.59 x 10-19 Coulombs. • This is less than 1% lower than the value accepted today: 1.602 x 10-19 C

30. Ernest Rutherford: 1903 • Rutherford studies under Thomson. • He discovered 3 types of natural radiation or radioactive decay. α - Alpha Particles β - Beta Particles γ - GammaRays high energy X-rays

31. Rutherford’s Gold Foil Experiment 1909 • This experiment showed the atom has a small, central positive nucleus and that most of the atom is emptyspace. Gold Foil Experiment on You Tube Rutherford Video Clip E drive You Tube: Discovery of the Nucleus 15 min

32. Rutherford’s Gold Foil Experiment • Used a narrow beam of  particles to bombard targets made of thin sheets of gold. Metal foil was surrounded by a fluorescent screen. • Results: • most of the  particles passed through the foil • some were deflected at small angles • few were deflected at large angles

33. View of the atoms in the Gold Foil Experiment • Rutherford's Gold Foil Experiment • Conclusions: • atom must contain a very small, dense center of positive charge • NUCLEUS • all the positive charge and 99.9% of the mass is in the nucleus • electrons define the space of an atom • electrons move at high speeds around the nucleus • atom does not have uniform density

34. Gold Foil Experiment on You Tube

35. Rutherford: 1909 • After his Gold Foil Experiment, Rutherford modifies his model of the atom to contain 2 basic regions: a small dense positive nucleus (protons) with electrons outside. • Proposed a neutral part of the nucleus

36. Neils Bohr: 1913 • Thought the atom was like the solarsystem (planetary model). Electrons orbit the nucleus with a fixed energy. • Energy Levels - analogous to rungs of a ladder • He wins the Nobel Prize for this model in 1922. It was eventually shown to be inaccurate and too simplistic.

37. Henry Moseley: 1913 • Worked under Rutherford. • Using CRT’s he bombarded metals with electrons and observed the emitted X rays by the metals • Results: each metal produced X rays of unique frequencies or wavelengths (X ray spectral lines)

38. Moseley cont. • Conclusions: He determined that each element has a unique nuclear charge. Hence, a different number of protons (Atomic Number). • Each atom is electrically neutral and therefore has an equal number of electrons. • Killed by a sniper in WW in 1915

39. James Chadwick: 1932 • Studied under Rutherford. • 1st isolated a neutron by bombarding beryllium atoms with alpha particles • He determined that the atom also contained a neutron which had approximately the same mass as a proton • Mass of proton = 1.673x10-24g • Mass of neutron = 1.675x10-24g • He proposed that the neutron had a neutral charge Chadwick won the Nobel Prize for his work in 1935.

40. Wave (electron cloud) Model:1924 to Present • Using QuantumMechanics, the electron can be found in a probability region.

41. FUN SONG The atom through the ages… The Atom Song By Michael Ouffutt To sum it up: Crash Course on the History of the Atom

42. Therefore: • There are 3 subatomic particles: protons, neutrons and electrons. These are measured in “atomic mass units” (amu) as their mass is so small.

43. Atomic Number and Mass Number • Atomic Number = the number of protons • Unique to each element • In a neutral atom, the number of protons equal the number of electrons. • Mass Number equal to the total number of protons + neutrons in the nucleus of an atom. Ex) carbon-12

44. Isotopes Atoms that have the same number of protons but a different number of neutrons (mass.) Isotopic Notation Shorthand way of representing an isotope of an element. Ex) top number is the mass number (#p + #n) bottom number is the atomic number (#p) May also be written: chlorine-37 or Cl-37 The actual average atomic mass for all chlorine isotopes is 35.45 amu

45. Isotopes of Hydrogen a. hydrogen (hydrogen – 1) 1p+ 0n0 b. deuterium (hydrogen – 2) 1p+ 1n0 c. tritium (hydrogen – 3) 1p+ 2n0

46. Ions • Formed when an atom gains or loses an electron a. Charge = # of protons - # of electrons Ex) Mg +2 = lost 2 electrons # of protons: 12 # of electrons: 10 Charge: +2 Positively Charged ion - CATION Ex) N-3 = gained 3 electrons # of protons: 7 # of electrons: 10 Charge: -3 Negatively Charged ion - ANION

49. CATION “cat”ion ca+ion ANION “ant”ion