Lecture4 bonds

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Geo 3500 - Lecture 4

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  • Lecture4 bonds

    1. 1. Bonds in Crystals
    2. 2. Bonds in CrystalsResults from the redistribution of e- that leads to a morestable configuration between two or more atomsThe valence e- are those involved in bondingWhen the e- have reorganized themselves, and if theenergy configuration is lower, then the atoms will staytogetherElectrical forces are chemical bonds, i.e., the attraction of+ and - charges
    3. 3. Chemical bondsCan belong to one of five principal bond types:‣ ionic‣ metallic‣ covalent‣ van der Waals‣ hydrogenIonic, covalent, and metallic bonds involve valence e-,while van der Waals and hydrogen bonds do not.
    4. 4. Ionic Bonds Ionic bond: forms when one or more e- in the valence shell of an atom are transferred to the valence shell of another, so that both elements achieve an noble gas configuration
    5. 5. Ionic Bonds
    6. 6. Na+ Cl- An idealized structure image with ionic bonding. The halite structure shown here represents bonding between Na and Cl.
    7. 7. Na+ Cl- An idealized structure image with ionic bonding. The halite structure shown here represents bonding between Na and Cl.
    8. 8. Ionic Bonds: Halite
    9. 9. Ionic BondThe attraction between oppositely charged ionsconstitutes the ionic (or electrostatic) bondThis bond forms as the result of the exchange of e-(s) of the metal atom to the nonmetal atom
    10. 10. Ionic BondIonic bonds commonlyform between atoms ofcolumns I and VII andcolumns II and VIAs the distance (due tothe size of the anion)increases, the bondstrength decreases, as afunction of increasinginter-ionic distances fig. 3.13
    11. 11. Ionic Bonds - Summary are generally of moderate hardness and have fairly high melting points are strong when forced together, but weak when cleaved or sheared are poor conductors of electricity and heat the symmetry of the resultant crystals is generally high
    12. 12. Metallic Bonds Metallic bond: the attractive force between positively charged nuclei with filled e- orbitals and the cloud of negative e- that holds such crystal structures together
    13. 13. Metallic Bonds
    14. 14. a) Schematic cross sectionfig. 3.15 through the structure of a metal. Each circle with a positive charge represents a nucleus with filled, nonvalence e- orbitals of the metal atoms. The mobile e- are represented by the cloud around the atoms (light gray). A possible e- path between the nuclei is shown by the line. b) An electron density map of copper atoms in copper metal showing the spherical nature and packing of the positively charged nuclei (white circles) surrounded by a less dense cloud of e- (contour lines)
    15. 15. Covalent Bonds Covalent bond: when two (or more) atoms share their outer valence e- it is an intermediate bond type, between ionic and metallic
    16. 16. Covalent BondsThe force of the bond is derived from the mutualsharing of e-Involves the merging and overlap of e- orbitals toachieve an octet configuration Cl- Cl-
    17. 17. Covalent BondsThe force of the bond is derived from the mutualsharing of e-Involves the merging and overlap of e- orbitals toachieve an octet configuration Cl- Cl-
    18. 18. Covalent Bonds
    19. 19. Covalent Bonds
    20. 20. Covalent Bonds
    21. 21. Covalent BondsElements near the middle of the periodic table, suchas C, Si, Al, and S, have 2, 3, and 4 vacancies intheir outer orbitals# of covalent bonds = # of shared e-The energy of the bonds produces a very rigidstructure - it is the strongest of all the chemical bonds
    22. 22. Covalent Bonds insoluble - generally slow reactivity chemical and mechanical stability very high melting points nonconductors of electricity the ions are no longer spherical, and the symmetry of the resulting crystals is likely to be lower
    23. 23. Electronegativity Ability of an atom in crystal structure or molecule to attract e- into its outer shell EN can be used as a basis for distinguishing elements: ‣ metals: < 1.9 ‣ metalloid: 1.9 < X < 2.1 ‣ non-metals: > 2.1
    24. 24. Electronegativity fig. 3.20
    25. 25. ElectronegativityAn atom whose EN exceeds that of the other atomby 2 or more, will generally be able to attract valencee- from the other atomIf not a lot of contrast - nearly equal EN - neitheratom is able to capture an e- from the other; theyshare
    26. 26. Bonds without Valence e-Van der Waals bond:force of the bond isderived from residualsurface charges on anotherwise relativelyneutral structure
    27. 27. Van der Waals bonds
    28. 28. van der Waals BondCould be considered as a weak dipole effectA small concentration of positive charge are oneend, leads to a small concentration of negativecharge at the other:
    29. 29. van der Waals Bond effective bonding over large distances in molecular structures generally defines a zone of cleavage and low hardness it is the weakest of the chemical bonds
    30. 30. Bonds without Valence e- Hydrogen bond: an electrostatic bond between a positively charged hydrogen ion and a negatively charged ion, such as O2- or N3- the hydrogen bond is considerably stronger than the van der Waals bond
    31. 31. Hydrogen bondThese bonds are weak, but there are many of them perunit volume of structure, which results in an overall,relatively strong materialBonding is common in hydroxides (OH-)Also present in many of the layer silicates, such asmicas and clay minerals
    32. 32. Bonds in Crystals In general, the stronger the average bond, the harder the crystal and the higher its melting point Hardness, cleavage, fusibility, electrical and thermal conductivity, and compressibility are all directly related to bond strength

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