Chemistry Reference
In-Depth Information
in each molecule because there are too many. Therefore, these formulas give only
the simplest ratio of atoms of one element to any others present. For example, the
formula for both diamond and graphite is C and that for silica is SiO
2
.
Systematic Method for Drawing Electron Dot
Diagrams
Drawing electron dot diagrams for some compounds or ions can get complicated.
If a diagram in which each nonmetal atom except hydrogen has an octet cannot
be made by using only single bonds, move pairs of unshared electrons to posi-
tions between atoms, forming double or triple bonds. However, for compounds or
ions that obey the octet rule, a more systematic approach may be used. We will
use PSCl
3
, in which the phosphorus atom is the central atom, as an example.
Steps
Example
S
Cl P Cl
Cl
Step 1:
Determine the arrangement of the atoms.
Step 2:
Determine the total number of valence elec-
trons
available
from all the atoms in the for-
mula unit.
1 P
1
5
5 valence electrons
1 S 1
6
6 valence electrons
3 Cl 3
7
21 valence electrons
total
32 valence electrons
Step 3:
Determine the total number of electrons
re-
quired
to get 8 electrons around each nonmetal
atom except hydrogen and 2 electrons around
each hydrogen atom.
1 P
8 electrons
1 S
8 electrons
3 Cl
24 electrons
total
40 electrons
Hydrogen atoms need only 2 electrons in their
outermost shells, and most main group metal ions
need none.
Step 4:
Subtract the number of electrons available from
the number required to determine the number of
shared electrons. (The shared electrons are
counted for each atom; that is, they are counted
twice to obtain the total number of electrons
needed.)
40
32
8 electrons to be shared
S
Step 5:
Distribute the shared pairs
between
adjacent
atoms.
Cl
P
Cl
Cl
Step 6:
Distribute the rest of the electrons to positions
other than between atoms, making sure that the
number of electrons required for each atom
(step 3) is now present.
Each atom now has 8 electrons:
S
Cl
P
Cl
Cl
