MOLECULAR COMPOUNDS
- compounds composed of non-metals only
- they are non-electrolytes because they are
made of molecules, not ions.
- the non-metal atoms move close enough
together that they can share electrons in
their valence level (outershell).
- each atom gets a share of 8 valence
electrons to make an octet.
- sharing of electrons makes a covalent bond.
H
H
C
H
H
H
H
O
N
H
methane
CH4
H
water
H2O
ammonia
NH3
H
TO PREDICT FORMULAS OF MOLECULAR
COMPOUNDS:
Follow the same rules as ionic compounds.
Example: carbon combining with oxygen
STEP 1: Write element symbols and valence
STEP 2: Make the number of positives equal to
the number of negatives
STEP 3: Reduce the subscript to lowest terms,
omit any subscript of 1
CO2
is carbon dioxide
- the carbon shares 4 electrons, each oxygen
shares 2 of their electrons
O
C
O
double bonds (2 pairs of electrons each)
Non-metal atoms may combine in different
ways with each other. This results in molecular
formulas that we wouldn’t normally predict.
Ex. CO Carbon monoxide
- the carbon shares 2 of its electrons, the
oxygen shares 4 of its electrons
O
C
triple bond (3 pairs of electrons)
NAMING MOLECULAR COMPOUNDS
Some molecular compounds have common
names:
H2O NH4 CH4 But there is a proper names for all molecular
compounds.
The method is similar to that for ionic
compounds. However, when the formula
contains more than one atom of a non-metal,
we add a prefix indicate the number of atoms.
The ending of the second non-metal is changed
to “ide”.
The prefixes used are:
number prefix number
1
mono
6
2
di
7
3
tri
8
4
tetra
9
5
penta
10
prefix
hexa
hepta
octa
nona
deca
Note: The prefix “mono” is omitted at the
beginning of all compound names,
except for Carbon Monoxide.
Examples:
H2O NH4 CH4 Try these:
i)
PCl5
ii)
SF2
iii)
N2O5
WRITING FORMULAS FOR MOLECULAR
COMPOUNDS
Ex. Sulfur trioxide
STEP 1: Write the symbols for each element in
the same order as in the name
STEP 2: Add subscripts to any symbol who’s
name has a prefix. DO NOT REDUCE TO
LOWEST TERMS.
Try these:
i)
carbon disulfide
ii)
carbon tetrachloride
iii)
sulfur difluoride
iv)
dihydrogen monosulfide
v)
hydrogen monosulfide
DIATOMIC MOLECULES
- molecules that exist as two atoms bonded
together.
Many non-metal atoms exist as diatomic
molecules instead of single atoms (Noble
gases are all monatomic)
The molecule is still named using the
element’s normal name. Prefixes are not used.
ALL HALOGENS ARE DIATOMIC MOLECULES
F2 – fluorine
Br2 – bromine
Cl2 – chlorine
I2 - iodine
ALL GASES AT ROOM TEMPERATURE
(OTHER THAN THE NOBLE GASES) ARE
DIATOMIC.
H2 – hydrogen
O2 – oxygen
N2 – nitrogen