Unit 15
Molecular Compounds
•Lewis Structures for Molecular Compounds and Ions
•Common Polyatomic Ions
•Nomenclature for Molecular Compounds
Lewis Structures for Molecular Compounds and
Ions (4.6, 4.10)
•Nonmetals – those to the right of the stair-step line
can react with each other by sharing electrons
•The driving force is similar to that for ionic
compounds – the attainment of an octet of electrons
as summarized in the Octet Rule
•In molecular compounds, though, the atoms do not
form individual ions, they share electrons throughout
their structure
•In this unit, we will look at drawing Lewis structures
representing these molecular compounds as well as
other aspects of these shared electron systems
Key Aspects of Lewis Structures (4.10)
•Key elements of a Lewis structure for a molecular
compound include:
•The number of valence electrons indicated equals the
total number of valence electrons in the compound or ion
•Electrons are shared in such a way that each atom is
effectively surrounded by eight electrons (two for
hydrogen, other exceptions exist).
•All or some of the electrons around an atom may be
shared with other atoms in the structure
An Approach to Drawing Lewis Structures (4.10)
•Several approaches exist for drawing Lewis structures –
this is just one you might consider:
1.
2.
3.
Draw skeletal structure for the atoms. Things that might be helpful
are hydrogen can only take one bond so it has to be on the outside;
carbon typically has four bonds, nitrogen has three bonds, and
oxygen has two bonds; polyatomic molecules and ions typically have
a central atom surrounded by other atoms.
Connect each atom to its neighbor(s) with a single bond which
consists of two electrons. The bond may be characterized by two
electrons (:) or a single line (—) attaching the two atoms.
Place electrons around the outside atoms so each has eight total
electrons including all of those bonded to it as well as the electrons
not bonded.
(Continued on Next Slide)
An Approach to Drawing Lewis Structures
Continued (4.10)
4. Count the number of valence electrons the structure should have.
This is the sum of the valence electrons on all atoms. For ions,
the number of valence electrons is increased by the charge and for
negative ions and reduced by the charge for positive ions.
5. Count the number of valence electrons in your structure.
6. If the valence electron counts match from steps 4 and 5 you are
done. If there are too many valence electrons in your structure,
take away a nonbonded pair and slide a nonbonded pair from an
adjacent atom to make a double chemical bond. Continue this
process until the number of valence electrons in the structure
matches the number that should be present.
Examples of Drawing Lewis Structures (4.10)
Draw the Lewis structure for CCl4
Step
1. Draw the skeletal structure
Progress
Cl
Cl
2. Connect atoms by a single bond
3. Add electrons to each atom to total eight electrons – notice
the there are two electrons in each bond
4. Count up the valence electrons you should have = 4 for C and
7 each for 4 Cl equals 32 valence electrons
5. Your structure has 32 valence electrons – it is done
C Cl
Cl
Cl
│
Cl — C — Cl
│
Cl
..
:Cl:
│
:Cl — C — Cl:
¨
│
¨
:Cl:
¨
Examples of Drawing Lewis Structures (4.10)
Draw the Lewis structure for NH3
Step
1. Draw the skeletal structure
Progress
H
H N H
2. Connect atoms by a single bond
H
│
H — N — H
3. Add electrons to each atom to total eight electrons – notice
the there are two electrons in each bond
H
│
H — N — H
¨
4. Count up the valence electrons you should have = 5 for N and
1 each for 3 H equals 8 valence electrons
5. Your structure has 8 valence electrons – it is done
Examples of Drawing Lewis Structures (4.10)
Draw the Lewis structure for SO2
Step
1. Draw the skeletal structure
2. Connect atoms by a single bond
3. Add electrons to each atom to total eight electrons – notice
there are two electrons in each bond
4. Count up the valence electrons you should have = 6 for sulfur
and 6 for each of 2 oxygens = 6 + 2 x 6 = 18 electrons
5. Your structure has 20 valence electrons – it needs to lose two
while electrons to get down to the 18 from Step 4. Remove any
two nonbonded electrons and, to maintain the octet, slide two
from nonbonded electrons from an adjacent atom into the bond.
6. Now there is the correct number of valence electrons and
each atom is surrounded by 8. Bonds count as two each and
nonbonded electrons count one each.
Progress
O S O
O — S — O
..
..
..
: O — S — O:
¨
¨
¨
..
: O ═ S — O:
¨
¨
¨