Chapter 16 Notes, part IIMore 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:

H2O

CO2

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?

H2O HCN CS2

NH3 CH2O

SiO2

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

• 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

• PCl5• SF6

• BF3