The Nuclear Atom

1. The Nuclear Atom Atomic Scientists’ Song 2:52

2. You need to understand and be able to explain the “evolution” of the atom. No names or dates are necessary

3. Aristotle (460 B.C. – 370 B.C.) • emphasized that nature consisted of four elements: air, earth, fire, and water • did not believe in discontinuous or separate atoms, but felt that matter was continuous

4. Democritus (460 B.C. – 370 B.C.) • first to suggest the existence of “atoms” (Greek word “atomos” = indivisible) • atoms are indivisible and indestructible • no experimental support http://www.stenudd.com/myth/Greek/images/democritus_1628_Brugghen.jpg

5. John Dalton (1766-1844) • used scientific method to test Democritus’s ideas • Dalton’s atomic theory • elements composed of atoms • atoms of the same element are alike • different atoms can combine in ratios to form compounds • chemical reactions can occur when atoms are separated, joined, or rearranged (but atoms are not created nor destroyed)

6. J.J. Thompson (1856-1940) • discovered the electron • thought atom was negative charges stuck in a positive charged lump • referred to as the “plum pudding model”

7. Robert A. Millikan (1868-1953) • found the quantity of charge carried by an electron (one unit of negative charge) • calculated the mass of an electron (1/1840th the mass of a hydrogen atom)

8. Ernest Rutherford (1871-1937) “Like howitzer shells bouncing off of tissue paper!” • proposed that the atom is mostly empty space • positive charges and almost all of the mass are in a small, centralized region called the nucleus

9. Rutherford Flash Animation

11. Try it Yourself! In the following pictures, there is a target hidden by a cloud. To figure out the shape of the target, we shot some beams into the cloud and recorded where the beams came out. Can you figure out the shape of the target? ?

12. The Answers Target #1 Target #2

13. Niels Bohr (1855-1962) • electrons found only in specific circular paths (orbits) around the nucleus • based on information about how the energy of an atom changes when it absorbs and emits light • called these fixed energies “energy levels”

15. Erwin Schrodinger (1926) • quantum mechanical model • probability of electron locations around the nucleus • not an exact orbit • eventually became the electron cloud model

16. Schrödinger's Cat video 1:41

17. Werner Heisenberg (1927) • Heisenberg Uncertainty Principle – impossible to know the exact position and momentum of an electron at the same time • “the observer affects the observed” http://www.deutsches-museum-bonn.de/ausstellungen/heisenberg/bilder/heisenberg_2.jpg

23. Structure of the Atom

26. Carbon- 12 as a standard • carbon- 12 • ALL masses on the periodic table are based on their relationship to carbon-12 • the carbon- 12 isotope has been given the atomic weight of exactly 12.000000000 and is used as the basis upon which the atomic weight of other isotopes is determined

27. He Even smaller particles • quarks • make up protons & neutrons • 6 types

28. Isotopes

31. Learning Check An atom has 14 protons and 20 neutrons. A. Its atomic number is 1) 14 2) 16 3) 34 B. Its mass number is 1) 14 2) 16 3) 34 C. The element is 1) Si 2) Ca 3) Se D. The number of electrons in a neutral atom is 1) 14 2) 6 3) 20

32. same element but differ in their number of neutrons • the atomic mass on periodic table is the WEIGHTED AVERAGE MASS of “all” the isotopes of that element • this is based on an isotope’s natural abundance • the percentage of each isotope of an element that occurs in nature • isotopes have the same chemical properties (reactivity) but different physical properties (density, melting/boiling point…)

33. http://education.jlab.org/itselemental/iso030.html

36. 2.3

38. X Mass Number Element Symbol Chemical symbols for isotopes • two different ways to write isotopes • example for sodium • sodium- 23 • only shows mass number (23) of the sodium isotope • 23 Na • shows the mass number (23) and the atomic # (11) of the sodium isotope 11 A Atomic Number Z

39. Isotopes? Which of the following represent isotopes of the same element? Which element? 234 X234 X235 X238 X 92939292 92 is the element uranium

40. How many protons, neutrons, and electrons are in-- How many protons, neutrons, and electrons are in-- 14 11 C C 6 6 Do You Understand Isotopes? 6 protons, 8 (14 - 6) neutrons, 6 electrons 6 protons, 5 (11 - 6) neutrons, 6 electrons

41. Radioactive Isotopes (don’t need to know this) • unstable isotopes that break down over time • uses: • cobalt 60 • radiation treatment for cancer • carbon 14 • used to date objects up to 60,000 years old • iodine 125 and iodine 131 • ingested and used for medical imaging Dangerous, but worth the risk

42. The Mass Spectrometer

44. has many applications, but one of the simplest is to determine the natural abundances of the isotopes of a particular element • the relative atomic mass can be calculated from the data from the mass spectrometer Mass spectrometer video (2:26) http://www.youtube.com/watch?v=_L4U6ImYSj0

46. Calculate the relative atomic mass of magnesium with the provided data. • magnesium results from the mass spectrometer: • 80% 24Mg • 10% 25Mg • 10% 26Mg • just a simple weighted mean • .80(24) + .10(25) + .10(26) = 24.3 amu