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 2O
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 (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.
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?
NH3
CH2O
SiO2
H2O
HCN
CS2
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