Section 5.2—
Drawing Molecules
Objectives:
•use the periodic table to identify the number of valence
electrons in an atom
•draw Lewis Dot structures for ionic and covalent
compounds
•introduce the idea of isomers
Drawing Molecules on Paper
Lewis Structures (or Dot Structures) are
one way we draw compounds on paper
Since paper is 2-D and compounds aren’t,
it’s not a perfect way to represent how
atoms bond…but it’s a good way to begin
to visualize compounds
Drawing Ionic Compounds
How do we determine the number of valence
electrons in an atom?
The number found with the “A” in the group
number IS the number of valence electrons.
1
2
3
4
5
6
7
8
Remember:
Put electrons on each side of the
element’s symbol.
Put one in each spot before doubling up!
Practice: Oxygen
Oxygen is in group VIA or 6A.
There are 6 valence electrons.
Example:
Draw the
Lewis
Structure for
an oxygen
atom
O
To Draw Ionic Compounds
Draw each atom
Transfer electrons from metal atoms to
non-metal atoms, keeping track of their
new charge
Practice: KCl
Potassium has 1 electron
Chlorine has 7 electrons
Example:
Draw the
Lewis
Structure for
KCl
K
Cl
Example:
Draw the
Lewis
Structure for
KCl
K
+1
Cl
-1
Add more atoms if needed
Example:
Draw the
Lewis
Structure the
ionic
compound of
Barium
fluoride
Barium has 2 electrons
Fluorine has 7 electrons
Example:
Draw the
Lewis
Structure the
ionic
compound of
Barium
fluoride
Ba
F
The fluorine is full, but the Barium isn’t!
Another fluorine atom is needed!
Example:
Draw the
Lewis
Structure the
ionic
compound of
Barium
fluoride
Ba
F
F
Example:
Draw the
Lewis
Structure the
ionic
compound of
Barium
fluoride
Now all have full valence shells
and the charges are balanced, just
as when you learned to write in
BaF2 in Chapter 2!
+2
Ba
-1
F
-1
F
A note about Ionic Dot Structures
The atoms are not sharing the electrons—
make sure you clearly draw the atoms
separate!
Practice Problems
Draw Lewis Dot Structures for compounds
that form between the following atoms.
What are the formulas for the compounds?
1.Al and Cl
2.Ba and O
3.Na and P
Drawing Covalent Compounds
Arrange the Atoms
 Atoms found in the middle of covalent
compounds are the atoms with the
LEAST electron affinity.
 Hydrogen & Halogens (F, Cl, Br, I) can
only bond with one other atom—they
can’t go in the middle of a molecules
 Always put them around the outside
 In general, write out the atoms in the
same order as they appear in the
chemical formula
Next:
 Use the periodic table to decide how
many electrons are around each atom
 Write the electrons around each atom
Practice: CH4
Carbon is in the middle – the H’s are
around it.
Carbon has 4 electrons
Each hydrogen has 1
Example:
Draw the
Lewis
Structure for
CH4
H
H C H
H
 Any electron that is being shared between
two atoms gets to be counted by both atoms!
 All atoms are “full” with 8 valence electrons
(except H—it can only hold 2)
 Each pair of electrons shared by two atoms
forms a bond
 A bond can also be represented by a single
dash
H
H C H
H
Sometimes . . .
 Sometimes, you don’t end up having a
full valence shells for all atoms in the
compound.
Practice: CH2O
•C has to go in the middle – the O and 2 H’s are
around it.
•C has 4 electrons, oxygen has 6 and each H has 1.
Example:
Draw the
Lewis
Structure for
CH2O
H
H C O
 Note: the carbon and oxygen only have 7 each
 However, they each have a single, unshared
electron.
 They could share those with each other!
Example:
Draw the
Lewis
Structure for
CH2O
H
H C O
Now the carbon and oxygen both have
8 valence electrons! (And the H’s have 2!)
Example:
Draw the
Lewis
Structure for
CH2O
H
H C O
Double Bonds
 Double bonds are when 2 pairs of
electrons are shared between the same
two atoms
Double Bond
Lone Pairs
 Lone pairs are a pair of electrons not
shared—only one atom “counts” them
Lone pair
And when a double bond isn’t enough…
 Sometimes forming a double bond still
isn’t enough to have all the valence
shells full
Practice: C2H2
 Both C’s go in the middle and 1 H goes
on each side.
 The C’s have 4 electrons each; the H’s
have 1.
Example:
Draw the
Lewis
Structure for
C2H2
H C C H
 Each carbon atom only has 7
electrons…not a full set of 8!
 But they each have an un-paired electron
left!
Example:
Draw the
Lewis
Structure for
C2H2
H C C H
 Each carbon atom only has 7
electrons…not a full set of 8!
 But they each have an un-paired electron
left!
Example:
Draw the
Lewis
Structure for
C2H2
H C C H
Now they each have 8 electrons!
Triple Bonds
 A triple bond occurs when two atoms
share 3 pairs of electrons
H C C H
Triple Bond
Properties of multiple bonds
Single Bond
Shorter bonds (atoms closer together)
Double Bond
Triple Bond
Stronger bonds (takes more
energy to break)
Practice Problems
Draw Lewis dot structures for:
1.H2O
2.O2
3.HCN
Special Notes
Polyatomic Ions
 They are a group of atoms bonded
together that have an overall charge
 Polyatomic ions have a covalent bond
within themselves…
 But an ionic bond with other ions
Isomers
For many compounds, there is more than
one correct way to draw a Lewis Dot
Structure
H C C C H
H
H
H
H C C
H
Chemical Formula: C3H4
Contains 2 sets of double bonds
between carbons
Both structures have full valence shells!
C H
Chemical Formula: C3H4
Contains 1 triple bond and 1
single bond between carbons
Both are “correct”
The chemical formula alone does not give
you enough information to differentiate
between the two structures
H C C C H
H
H
H
H C C
H
C H
Chemical Formula: C3H4
Isomers
Isomers: Structures with the same
chemical formula but different chemical
structure
Atoms can be bonded differently (multiple
versus single bonds) or in a different
order) but have the same overall chemical
formula in isomeric structures.