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CHEMICAL BONDING

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CHEMICAL BONDING

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    1. CHEMICAL BONDING Cocaine To play the movies and simulations included, view the presentation in Slide Show Mode. To play the movies and simulations included, view the presentation in Slide Show Mode.

    2. Chemical Bonding Problems and questions — How is a molecule or polyatomic ion held together? Why are atoms distributed at strange angles? Why are molecules not flat? Can we predict the structure? How is structure related to chemical and physical properties?

    3. Review of Chemical Bonds There are 3 forms of bonding: _________—complete transfer of 1 or more electrons from one atom to another (one loses, the other gains) forming oppositely charged ions that attract one another _________—some valence electrons shared between atoms _________ – holds atoms of a metal together

    4. The type of bond can usually be calculated by finding the difference in electronegativity of the two atoms that are going together.

    5. Electronegativity Difference If the difference in electronegativities is between: 1.7 to 4.0: Ionic 0.3 to 1.7: Polar Covalent 0.0 to 0.3: Non-Polar Covalent

    6. Ionic Bonds All those ionic compounds were made from ionic bonds. We’ve been through this in great detail already. Positive cations and the negative anions are attracted to one another (remember the Paula Abdul Principle of Chemistry: Opposites Attract!)

    7. Electron Distribution in Molecules Electron distribution is depicted with Lewis (electron dot) structures This is how you decide how many atoms will bond covalently! (In ionic bonds, it was decided with charges)

    8. Bond and Lone Pairs Valence electrons are distributed as shared or BOND PAIRS and unshared or LONE PAIRS.

    9. Bond Formation A bond can result from an overlap of atomic orbitals on neighboring atoms.

    10. Review of Valence Electrons Remember from the electron chapter that valence electrons are the electrons in the OUTERMOST energy level… that’s why we did all those electron configurations! B is 1s2 2s2 2p1; so the outer energy level is 2, and there are 2+1 = 3 electrons in level 2. These are the valence electrons! Br is [Ar] 4s2 3d10 4p5 How many valence electrons are present?

    11. Review of Valence Electrons Number of valence electrons of a main (A) group atom = Group number

    12. Steps for Building a Dot Structure Ammonia, NH3 1. Decide on the central atom; never H. Why? If there is a choice, the central atom is atom of lowest affinity for electrons. (Most of the time, this is the least electronegative atom…in advanced chemistry we use a thing called formal charge to determine the central atom. But that’s another story!) Therefore, N is central on this one 2. Add up the number of valence electrons that can be used. H = 1 and N = 5 Total = (3 x 1) + 5 = 8 electrons / 4 pairs

    13. 3. Form a single bond between the central atom and each surrounding atom (each bond takes 2 electrons!)

    14. Check to make sure there are 8 electrons around each atom except H. H should only have 2 electrons. This includes SHARED pairs.

    15. Carbon Dioxide, CO2 1. Central atom = 2. Valence electrons = 3. Form bonds.

    16. Carbon Dioxide, CO2

    17. Double and even triple bonds are commonly observed for C, N, P, O, and S

    18. Now You Try One! Draw Sulfur Dioxide, SO2

    19. Violations of the Octet Rule (Honors only) Usually occurs with B and elements of higher periods. Common exceptions are: Be, B, P, S, and Xe.

    20. MOLECULAR GEOMETRY

    21. VSEPR Valence Shell Electron Pair Repulsion theory. Most important factor in determining geometry is relative repulsion between electron pairs.

    22. Some Common Geometries

    23. VSEPR charts Use the Lewis structure to determine the geometry of the molecule Electron arrangement establishes the bond angles Molecule takes the shape of that portion of the electron arrangement Charts look at the CENTRAL atom for all data! Think REGIONS OF ELECTRON DENSITY rather than bonds (for instance, a double bond would only be 1 region)

    26. Structure Determination by VSEPR Water, H2O

    27. Structure Determination by VSEPR Ammonia, NH3 The electron pair geometry is tetrahedral.

    28. Bond Polarity HCl is POLAR because it has a positive end and a negative end. (difference in electronegativity)

    29. This is why oil and water will not mix! Oil is nonpolar, and water is polar. The two will repel each other, and so you can not dissolve one in the other

    30. Bond Polarity “Like Dissolves Like” Polar dissolves Polar Nonpolar dissolves Nonpolar

    31. Diatomic Elements These elements do not exist as a single atom; they always appear as pairs When atoms turn into ions, this NO LONGER HAPPENS! Hydrogen Nitrogen Oxygen Fluorine Chlorine Bromine Iodine

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