Atoms and Elements 29 September 2005

1. Atoms and Elements 29 September 2005 Physics Chemistry Astronomy Geology Biology

2. Topics • Discovery of atom structure and function • Models of atoms • Bohr atom (1913) • Quantum mechanics (1930’s) • Elements and atoms • Electrons and orbits/shells • Valence electrons

3. Scientific Method Observations Rejection/Acceptance Hypothesis Testing Hypothesis

4. Models in the Sciences Example: Dynamics of family interactions Example: Structure of the atom (protons, neutrons and electrons Models are an abstraction and meant to be challenged Protons and Neutron Electrons in orbits

5. Crystals: Structure and Color

6. Compounds to Subatomic Particles: A Hierarchy Compound Element Atom Subatomic Particle Emergent Properties

7. Thompson’s Discovery of the Electron • Idea that there are smaller components that comprise atoms • Hypothesis: Atoms consist of subcomponents, one of which is negatively charged with a very small mass • Testing the hypothesis

8. Thompson’s Experiment

9. Rutherford’s Discovery of a Nucleus • Idea that there are other smaller components of atoms, including some that are positively charged • Hypothesis: Atoms consist of multiple subcomponents, some negative and some positive • Testing the hypothesis • Alpha particles (+), gold foil, “bullets”, and “tracks”

10. Rutherford’s Experiment:

11. Observations, Hypothesis and Conclusion • Observations • Almost all alpha’s passed through foil unaffected • Very small number of alpha’s deflected @ small angle • 1/1000 deflected at large angle (reflected back) • Hypothesis: Atom consists of a positive subcomponent that is very small in volume in comparison to atom itself • Accept/reject?

12. Atomic Structure: Bohr Atom • Observations (1913) • Heat hydrogen gas and light is emitted as a discrete wavelength (not continuous spectrum) • Other gases behave the same in producing discrete wavelength, but each gas is unique in wavelengths emitted

13. Hypothesis and Model

14. Model: Quantum Mechanics

15. Theory of Quantum Mechanics • Observations • Behavior of electrons in heated hydrogen gas consistent with Bohr Model (orbits, etc.) • Behavior of other heavier gases not explained by Bohr Model • Investigators (Heisenberg, Shrodinger): wave-particle duality of light • Key: integrated mechanisms of waves and particles, focusing on “fuzzy electron clouds”/waves • Hypothesis: new model of atomic structure and function: quantum mechanics theory of the atom • Incorporation of known facts • Prediction of new properties yet to be discovered

16. Questions • Is a model in the sciences a form of an hypothesis? • All of these models (when proven correct) resulted in scientific revolutions in physics. How are revolutions in the sciences analogous to revolutions in society?

17. Question According to the Rutherford model of the atom, the volume of any atom is largely _______. A. protons and neutrons B. electrons C. empty space D. covertly sequenced nuons surrounded by pompous protons

18. Question The existence of a tiny, massive, and positively charged atomic nucleus was deduced from the observation that ___. A. fast, massive, and positively charged alpha particles all move in straight lines through gold foil B. alpha particles were deflected by a magnetic field C. some alpha particles were deflected by metal foil D. all of the above

19. Question According to the Bohr model of the atom, an electron gains or looses energy only by ______. A. jumping from one atom to another B. speeding up or slowing down in its orbit C. jumping from one orbit to another D. being removed from the atom

20. Topics • Discovery of atom structure and function • Models of atoms • Bohr atom (1913) • Quantum mechanics (1930’s) • Elements and atoms • Electrons and orbits/shells • Valence electrons

21. Elements • Patterns • 92 naturally occurring elements (e.g., hydrogen, gold, helium) • Total of 113 • 25 of 92 are essential to life (e.g., what are they?) • Key points • any element is the same in its chemical structure and physical properties (~stable over time) • All elements have origin in either the big bang (hydrogen and helium) or the subsequent evolution of the universe

22. Elements • Compound • Elements combine in very precise ways that are recurrent and predictable • Sodium + Chlorine = Sodium Chloride Na + Cl = NaCl metal + gas = solid • Key points • Atoms of sodium (Na) and chlorine (Cl) remain atoms of each • Emergentproperty: “emergence” of new properties in a compound not be explained by the summation of the two elements (hierarchy theory)

23. Compounds to Subatomic Particles: A Hierarchy Compound Element Atom Subatomic Particle Emergent Properties

24. Particles • Atoms are composed of particles (subatomic particles) • Most stable particles • Neutrons • Protons • Electrons • Other less stable particles (quarks, neutrinos, etc.) • Relationship among the more stable particles NeutronProton Electron Charge neutral positive negative Mass 2x10-24 g 2x10-24 g 5x10-28 g

25. Atomic Structure • Atoms of the same element have the same number of subatomic particles, we abbreviate this as follows: 2 # of protons Helium He Abbreviation of element 4Atomic mass (g/mole) 1 Hydrogen H 1 • Mass = Protons + Neutrons

26. Electrons • Background of quantum mechanics • Energy “barons” of the atom (motion) • Energy = ability to do work • Potential energy = energy stored due to position or location • Charge is negative (-) and particle is always in motion • Capturing an atom and its orbiting electron may be difficult

27. Key to Electron Structure • Count the number of electrons (and compare with abbreviation of element) • Electrons are negative in charge and in constant motion • Electrons are in orbits around the nucleus

28. Electrons • Example of Sulfur (1632S) (16 electrons in 3 shells) What are shells and what are orbits?

29. Example of Electrons in Shells As electrons move among shells, they change potential energy • Hot summer day, bright sun and car top • Light absorption by pigments and electrons “jump” to higher shell (potential energy); give off energy when they drop back (kinetic energy) • Banana, orange juice or bagel this AM? • Excited electron “captured” by chlorophyll in leaf and shuttled to a sugar molecule in its excited state (potential energy) until you release the energy via digestion/respiration, allowing the electron to “drop back” to a lower orbit (kinetic energy)

30. Some keys to Electron Structure • Electrons reside in shells as a function of quantum mechanics (1-4 orbits per shell) • Never more than two electrons per orbit (Pauli’s Exclusion Principle) • Distribution of electrons is key to understanding why elements and atoms behave the way they do • Outermost electrons are called valence electrons and they have a very special significance in chemistry

31. Periodic Table of Elements • Concept: most stable state for an atom is one in which the outermost shell is filled with the maximum number of electrons • 1st Shell (1 orbit; 2 electrons) • Hydrogen (11H; 1 electron; stable ?) • Helium (24He; 2 electrons; stable ?) • Periodic Table’s 1st Row • Hydrogen and Helium

33. Periodic Table of Elements • 2nd shell has 4 orbits with 2 electrons (maximum) per orbit (total of 8 electrons/shell) • Most stable configuration is: • 1st shell filled with 2 electrons • 2nd shell filled with 8 electrons • Total of 10 electrons (1020Ne) • 2nd row of Periodic Table • 8 elements

34. Periodic Table of Elements • 3nd shell has 4 orbits with 2 electrons maximum per orbit (total of 8 electrons/shell) • Most stable configuration is the following: • 1st shell filled with 2 electrons • 2nd shell filled with 8 electrons • 3rd shell filled with 8 electrons • Total of ___ electrons (1840Ar) • 3nd row of Periodic Table • 8 elements (list and relate to the above)

36. Periodic Table of Elements • Number of elements in a row is not chance but reflects the maximum number of electrons in the outermost shell • Row 1 = 2 • Row 2 = 8 • Row 3 = 8 • Row 4 = 18 • etc

37. Discussion Question As you scan the night sky, you see multiple objects, and you question whether these objects are similar to or different from Earth. Suppose the person next to you says that she has an instrument that can identify the occurrence of specific atoms (e.g., sodium, hydrogen, sulfur) based on the energy patterns/signatures of electrons in atoms on that planet. Could she be right?

38. Discussion Question Models are one of the key investigative tools in the scientific method. Models of our understanding of the atom are a classic example of the role that models play in the sciences. List five features of models – not the specifics of any given model, but rather how models are constructed, how they serve to help promote our understanding of natural systems, and their fallibility.

39. Discussion Question In many ways, the Periodic Table is a “model” to help scientists understand patterns in the natural world. List ways in which this model helps scientists to understand the natural world? How does this model compare with that of Rutherford’s model of the atom or the quantum mechanics model of the atom?