The “Bag Method” for writing Covalent Lewis Dot Structures
1.
2.
3.
4.
5.
6.
7.
8.
Count all the valence electrons and adjust the number of electrons according to the
net charge.
Put all these electrons in a “bag.”
Arrange the atoms near each other, in a pattern sensible for bonding.
Put one shared pair of electrons between the central atom and each of the outer
atoms. Subtract these electrons from the bag.
Put lone pairs on the outer atoms to complete their octets. Subtract these electrons
from the bag.
Check the bag. If it still has electrons in it, use the remaining electrons to complete
the octet of the central atom.
Check the octet of the central atom; make multiple bonds, if necessary.
Check the total electron count and the octet rule.
Example:
Draw the Lewis Structure for SO2
1.
2.
S atom + 2(O atoms) + zero charge = 6 + 2(6) + 0 = 18 electrons.
18 electrons in the bag.
3.
ENS = 2.5, ENO = 3.5. Put S in the middle!
4.
O
O
S
O
S
_
O
_
_
_
_
_
_
_
Bag: 14 e - 2(6) e = 14 e -12 e = 2 e
O
S
O
6.
_
_
Bag: 2 e - 2 e = 0 e
_
_
_
NO MORE e .
Sulfur still has only six electrons. It needs two more. We must take a lone pair off
oxygen and move it to a bonded pair between the sulfur and the oxygen.
O
8.
_
S
Bag: 18 e - 2(2) e = 18 e -4 e = 14 e
O
5.
7.
O
S
O
The structure has 18 electrons (3 shared pairs and six lone pairs). Sulfur has eight
electrons (three shared pairs and one lone pair). The left oxygen has eight electrons
(two shared pairs and two lone pairs). The right oxygen has eight electrons (one
shared pair and three lone pairs).
Other examples: NH4+, HCN, H2CO
The “Bag Method” for writing Covalent Lewis Dot Structures
Lewis Dot Structure uses the concept of shared pairs of electrons and the octet rule to
illustrate how atoms can bond covalently to make a stable structure.
1.
2.
3.
4.
5.
6.
7.
Count all the valence electrons and adjust the number of electrons according to the
net charge.
a. Cations have fewer electrons than the total number of atomic valence electrons,
so take electrons away from the number of valence electrons.
b. Anions have more electrons than the total number of atomic valence electrons, so
add electrons to the number of valence electrons.
Put all these electrons in a “bag.”
a. Electrons will be removed from the “bag” as they are needed in the structure.
b. Shared pairs of electrons, or BONDED PAIRS, are represented by a line, —.
c.
Unshared pairs of electrons, or LONE PAIRS, are usually represented by two
dots, .
d. When you’re done, the “bag” should be empty and all the electrons that were in
the bag must end up on the structure either as lines (—) or dots ( ).
Arrange the atoms near each other, in a pattern sensible for bonding.
a. Put the least electronegative element in the middle of the group, since this
CENTRAL ATOM has to share electrons with all the other atoms.
b. Never make hydrogen the central atom, even if it has the lowest electronegativity. The central atom needs to be able to share many electron pairs.
c.
Put all the other atoms around the central atom. The OUTER ATOMS, in general,
have higher electronegativity, so they want to share as little as possible.
Put one shared pair of electrons between the central atom and each of the outer
atoms. Subtract these electrons from the bag.
Put lone pairs on the outer atoms to complete their octets. (Remember, hydrogen
atom does not need an octet. Its happy with two electrons!). Subtract these electrons
from the bag.
Check the bag. If it still has electrons in it, use the remaining electrons to complete
the octet of the central atom.
Check the octet of the central atom. Does it have a full octet of electrons?
a. If no, then make a multiple bond between the central atom and the least
electronegative outer atom by causing one of its lone pairs to become a bonded
pair. To do that: Erase one pair of dots, , on the outer atom and draw a line
(—) between the outer atom and the central atom.
b. WARNING: Do not make multiple bonds unless you need more electrons than
there are in the bag.
c.
WARNING: Never make double bonds to hydrogen.
8.
Check the total electron count and the octet rule. Does every atom have a full octet
of electrons? (Remember, hydrogen atom does not need an octet. It’s happy with two
electrons!)