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Chapter 16 Notes, part II

Chapter 16 Notes, part II. More on Covalent Bonding. Drawing Line Diagrams. A line diagram replaces the circled shared pair of electrons with a line. Lines must be straight and short; double bonds should look like an equal sign. Draw line diagrams for: H 2 O CO 2 HCN. Resonance.

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Chapter 16 Notes, part II

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  1. Chapter 16 Notes, part II More on Covalent Bonding

  2. Drawing Line Diagrams • A line diagram replaces the circled shared pair of electrons with a line. • Lines must be straight and short; double bonds should look like an equal sign. • Draw line diagrams for: H2O CO2 HCN

  3. Resonance • If you draw the covalent bonding diagram of some compounds—nitrite for example—there is not only one way to draw it. • There are two valid ways to show the structure of nitrite. The real structure is an average of the two, and this is called a resonance structure. It is shown by:

  4. Resonance, cont. • Earlier chemists thought the compound just quickly flipped back and forth between the two structures (or resonated). • This is proven to be untrue now using bond lengths.

  5. Coordinate Covalent Bonding • A coordinate covalent bond occurs when one atom gives both electrons to a shared pair between them. • In a line diagram, instead of a straight line showing a shared pair, the coordinate covalent bond is shown as an arrow.

  6. Sigma and Pi Bonds • How do covalent bonds form between two elements? • By the combining of their p orbitals.

  7. Sigma and Pi Bonds • The first bond between two atoms is a sigma bond (s) and it forms because of end-to-end overlap of p orbitals. • Any other bonds between the same atoms would be a pi bond (p) and they form because of side-to-side overlap of p orbitals.

  8. Sigma and Pi Bonds • Single bond = 1 sigma bond • Double bond = 1 sigma + 1 pi bond • Triple bond = 1 sigma + 2 pi bonds

  9. Sigma and Pi Bonds • How many sigma and pi bonds do the following compounds have? NH3 CH2O SiO2 H2O HCN CS2

  10. So, then how… • If bonding occurs only with p orbitals, how does carbon make 4 bonds? • By forming hybrid orbitals!

  11. Hybrid Orbitals • The s-orbital, which is not normally used for bonding, combines with the p-orbitals to make 4 equal energy bonding orbitals.

  12. Hybrid Orbitals • Some bigger elements will also form hybrid orbitals using their d-sublevel’s orbitals. • This allows for compounds that wouldn’t normally be able to form—compounds that break the octet rule.

  13. Exceptions to the Octet Rule • PCl5 • SF6 • BF3

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