#1 Count the total number of valence
electrons in the molecule
C: 4 electrons x 1 atom = 4 valence electrons
H: 1 electron x 4 atoms = 4 valence electrons
Total = 8 valence electrons
#2 Find the number of “octet”
electrons for the molecule.
C: 8 octet electrons x 1 atom = 8 octet electrons
H: 2 octet electrons x 4 atoms = 8 octet electrons
Total = 16 octet electrons
Rules:
Hydrogen always has 2 octet electrons
Beryllium always has 4 octet electrons
Boron wants 6 for neutral molecules (8 if it’s an anion)
#3 Subtract the number of valence
electrons from the number of
octet electrons to find the number
of bonding electrons.
16 – 8 = 8 bonding electrons
#4 Divide the number of bonding
electrons by 2 to find the number
of bonds in the molecule.
8 / 2 = 4 bonds in CH4
Why 2?
Because there are two electrons in every covalent bond!
#5 Draw an arrangement of atoms
that has the number of bonds you
found in step 4. Of course, there
are more rules…
Hydrogen and Halogens = bond once!
Oxygen’s family and Beryllium bond twice in neutral molecules
Nitrogen’s family and Boron bond 3 times in neutral molecules
Carbon’s family = bonds 4 times!
More hints…
If you bond everything together and
you have bonds left over, look for
double/triple bonds.
The atom nearest to the left side of
the periodic table is probably in the
middle of the molecule.
H2O
CCl4
NH3
CO2
PCL3
N2
CH4
C2H6
CH3OH
BH3
PBr3
N2H2
C2H2
SiH4
C2H4
HF
HCl
CCl2F2