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 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:

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.

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.

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

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

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

Sigma and Pi Bonds How many sigma and pi bonds do the following compounds have? H2OH2OHCNCS 2 NH 3 CH 2 O SiO 2

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

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

Hybrid Orbitals Carbon has three different ways it forms hybrid orbitals. sp 3 —1s and 3p orbitals combine so four single bonds can form sp 2 —1s and 2p orbitals combine, leaving 1p orbital to make a pi bond (allows double bond) sp—1s and 1p orbitals combine, leaving 2p orbitals to make either 2 double or 1 triple bond

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.

Exceptions to the Octet Rule PCl 5 SF 6 BF 3