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Atoms

Atoms. History, Theory, Structure. What’s In the Box?. How Do We Know Atomic Structure?. How can we describe something we can’t even see? Compare to Black Box Experiment Can’t see internal structure Discovery by experimenting with how matter behaves

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Atoms

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  1. Atoms History, Theory, Structure

  2. What’s In the Box?

  3. How Do We Know Atomic Structure? • How can we describe something we can’t even see? • Compare to Black Box Experiment • Can’t see internal structure • Discovery by experimenting with how matter behaves • Constantly changing and/or refining definition

  4. What is Inside an Atom? • Our current understanding of atomic structure is based on: Electron Shells (Not exact locations) e- Nucleus Contains Protons (+) and Neutrons (N) + N + Drawn as an outer “wall” only for reference; Energy

  5. Historical Perspective • Aristotle: matter may be continuously divided without changing its intrinsic properties • Democritus: matter is composed of tiny, indivisible particles; atoms may not be divided • Dalton: Atoms may not be further divided. • Thomson: pudding theory; electrons • Rutherford: gold foil • Bohr: quantum theory • Schrodinger: charge-cloud “Ancient” “Modern”

  6. Dalton • 1808 • English • Two very important tenets • Law of Conservation of Mass • Law of Definite Proportions

  7. Dalton’s Main Points • All matter is made up of atoms. • All atoms of an element are identical in size, mass, and chemical properties. • Atoms of an element are different from atoms of all other elements. • Atoms may neither be divided nor destroyed. • Atoms may be combined, separated, and rearranged to form new compounds. • Atoms of different elements combine in whole-number ratios to form compounds.

  8. Thompson

  9. The Nucleus • 1911 • Rutherford’s Gold Foil Experiment • Actually conducted by Rutherford’s assistant • Hugely advanced for time period

  10. Bohr and Quantum Theory • Bohr’s model related structure and position of electrons to energy levels • Ground state • Excited state • Line-emission spectrum • Continuous spectrum • Only worked consistently for hydrogen

  11. Charge-Cloud • Based on Bohr’s ideas with one main difference • Electrons are not traveling in fixed positions • Use of orbitals rather than orbits • Areas where electrons are most likely to be found • For example, an electron will never be expected to be found in the atom’s nucleus

  12. What’s Inside an Atom? • Protons have a positive charge. • Mass of 1 unit • Located in nucleus • Identity of an atom • Neutrons have no charge • Mass of 1 unit • Located in nucleus • Help to stabilize nucleus

  13. Electrons • Located outside of the nucleus • Negative charge • Mass is insignificant to mass of whole atom • Responsible for chemical reactivity of an atom • Though mass is tiny, equal but opposite charge • Can change position • Relate to energy of an atom

  14. Sub-Atomic Particles • Negative charge • Located in energy levels around nucleus • Responsible for chemical reactivity of an atom • Positive charge • Located in nucleus • Determine properties of an atom • Unique to each element • No charge (neutral) • Located in nucleus • Thought to provide stability to nucleus • Numbers may vary (isotopes) Protons Neutrons Electrons

  15. Atomic Mass • All mass measurements are comparisons based on standards • All atoms based on mass of carbon-12 atom • Weighted average of all known isotopes based on measured frequency • Combination of protons and neutrons

  16. Quantifying Atoms • Atomic Mass • represents combined masses of protons and neutrons • Atomic Number • Represents number of protons; in a neutral atom represents number of electrons

  17. Isotopes • While all atoms of an element have the same number of protons, numbers of neutrons may differ • These atoms still retain the basic properties of the element • Small differences in behavior • Atoms of the same element with differing numbers of neutrons • Indicated by symbol with number to indicate number of neutrons or mass

  18. The Periodic Table of the Elements: History • Mendeleev (1834-1907); Russian; 1880’s • listed approximately 73 known elements in columns in order of increasing atomic mass • left blank spaces for unknown elements • Moseley (1887-1915); British; worked w/ Rutherford; 1913 • determined atomic number of atoms of known elements • arranged elements by atomic number

  19. Modern Periodic Table • Periods • Horizontal • 7 • Periodic Law • Properties change as you go from left to right on the Periodic Table. • Atomic mass increases • Atomic number increases

  20. Modern PT, cont. • Group or Family • Vertical • Similar physical and chemical properties • Significance of Group Number • For main group elements represents number of valence electrons

  21. Metals • Approximately 80% of known elements • Alkali: Group I • Highly reactive • Alkaline Earth: Group 2 • Highly reactive • Transition, inner transition • Properties • malleable, ductile, lustrous • solids at room temp except Hg • good conductors of heat and electricity

  22. Nonmetals • Not lustrous • Poor conductors of electricity and heat • Halogens • Group 7 • Noble gases • Group 8 • Right side of Periodic Table

  23. Metalliods • Zigzag line • Properties of both metals and nonmetals • Used in computer chips and solar cells; semiconductors

  24. Periodic Table of the Elements Source: Jefferson Labs

  25. Summary • Historical perspective • Models of atoms based on behavior rather than a visual analysis • All atoms of an element have same basic properties • Isotopes • Atoms of same element with differing # of neutrons • Identity of an atom based on protons • Chemical reactivity of an atom based on electrons • Periodic Table of the Elements • Organization and Patterns

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