Everyone who has studied chemistry should be able to draw Lewis
structures. Although there are many complicated situations, and some
people try to stretch Lewis structures to be an accurate description
of molecules even when they don't work well, the basic idea is
simple. Here's how I draw Lewis structures.
You count the valence electrons. Count the valence electrons for
each atom, add them up, and add or remove electrons if there is an
overall charge.
You figure out what the connections between atoms are. Sometimes
you might look this up. Other times, you have to guess. If the
molecule is linear (like HCN) usually it is written in the correct
order. If it is a polyatomic ion, like sulfate or nitrate, usually
you put the heavy atom, or the atom to the left in the periodic
table, in the center. You should probably not put all the atoms in a
line if there are more than 4 (single-bonded chains are usually very
unstable, except for carbon). Elements like N, C, S, P, Cl and the
heavier elements in these groups can easily connect to 4 other
atoms, so often they go in the middle. O should not connect to more
than 2 atoms, and often only connects to one. If O connects to 2
atoms, usually at least one is C or H. H and F will almost
always make just one bond. (Hydrogen bonds, which you may have heard
of, are much weaker than the covalent bonds shown by Lewis
structures).
Once you have chosen an arrangement of atoms, add the right
number of electrons. Try to make sure every element gets the right
number of electrons, using lone pairs
of electrons which are not shared, or shared pairs (which are
bonds). You can draw single, double, or triple bonds. Make sure that
H has 2 electrons (never more) and C,N,O,F have 8 electrons (never
more, and not less unless the molecule has an odd number of
electrons). The heavy elements under C-F should have at least eight
electrons, and they can also connect to 6 or even 7 other atoms. B
often has 6 electrons, and Be often has 4. Move the electrons
around until it works. Make sure your final structure has the right
total number of electrons, and that none of the atoms have too many
or too few.
Unpaired electrons are called radicals,
and you should avoid them. When you draw the Lewis structure, make all the electrons paired
unless there is an odd number of electrons. All electrons should be
in lone pairs or bonding pairs. (There are molecules, like
O2, which have unpaired electrons even though they could
all be paired, but you can't predict that with Lewis structures, so
assume they are all paired.)
Acceptable numbers of electrons and connected atoms for
common elements
Element
Number of electrons (including
shared)
Number of connected atoms
Exceptions
H
2
1
sometimes connects to 2
atoms, in H-bonding or with boron
Be
0,4,6,8
2-4
can be shown as ionic
(0 electrons) or covalent (4, 6, 8)
B
6, 8
3, 4
sometime has less than 8
electrons
C
8
1-4
less electrons if compound
is a radical
N
8
1-4
less electrons if compound
is a radical
O
8
1-2
sometimes connects to 3
atoms, such as in H-bonding
F
8
1
sometimes connects to 2 atoms,
such as in H-bonding
P and below
8 or more
3-6
S and below
8 or more
2-6
Cl and below
8 or more
1-6
I can
connect to 7 atoms
Xe
8 or more
0-6
Xe compounds with O
and F are known
Here are 2 examples. First, let's do hydrogen cyanide, the poison
that might have killed Lewis. The formula is HCN. As usual, this is
the correct order of the atoms. The number of valence electrons in
the molecule is (1 + 4 + 5) = 10. When I'm putting the electrons in,
I usually start by putting each atom's valence electrons around it,
then I connect the dots into lines. (These steps are shown in the
picture.) In this case, if we put a single or double bond between the
C and N we will not be able to give both of them octet.
For a second example, let's do the tetrafluoroborate ion,
BF4–. In this case, we have to put B in
the middle, because F shouldn't make more than 1 bond. We count
electrons: (3 + 7 x 4 + 1) = 32. Remember to count +1 for the
negative charge on the ion. Because B needs to make 4 bonds, we'll
give it the extra electron. Then we'll connect the electrons into
bonds. In this case, you know that F pulls on electrons much harder
than B, so the "shared pairs" will probably be closer to F, even
though the picture doesn't show that.