Atoms

1. Atoms History, Theory, Structure

3. What’s In the Box?

4. 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

5. 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

6. 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”

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

8. 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.

9. Thompson

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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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

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

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

25. Periodic Table of the Elements Source: Jefferson Labs

26. 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