Chemistry
Boon
Date: 09/06 & 09/07 2012
NOTES: LEWIS DOT
STRUCTURES
Catalyst
Ionic or Covalent? Hint: Ionic bonds form between
a metal and a nonmetal. Covalent bonds form
between two or more nonmetals.
1. NaCl
2. MgF2
3. O2
4. H2
5. CO2
6. CH4
7. Ag2O
Objectives
 I can explain the difference between ionic and
covalent compounds and bonds.
 I can draw simple Lewis structures of
molecules.
Agenda
 Catalyst
 HW Review
 Open Note Quiz
 Lewis Dot Structures of Molecules
HW Review: p. 184 #23
Atom
Ion
Noble-gas configuration of ion
S – sulfur (6 v.e.)
S2-
Argon – 1s2 2s2 2p6 3s2 3p6
Be – Beryllium (2 v.e.)
Be2+
Helium – 1s2
I – Iodine (7 v.e.)
I-
Xenon - 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6 5s2 4d10 5p6
Rb – Rubidium (1 v.e.)
Rb+
Krypton - 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6
O – oxygen (6 v.e)
O2-
Neon - 1s2 2s2 2p6
Sr – Strontium (2 v.e.)
Sr2+
Krypton - 1s2 2s2 2p6 3s2 3p6 4s2 3d10 4p6
F – Fluorine (7 v.e.)
F-
Neon - 1s2 2s2 2p6
Hint: Octet Rule: Metals lose all their negatively charged valence electrons
to form positive cations. Nonmetals gain enough electrons to have a full
outer shell (usually to end up with 8) and form negative anions.
•These ions end up with the same number of electrons as the noble gas
closest to them on the periodic table.
HW Review (p. 198 & 216)
 7. Molecular compounds have low melting points




and low boiling point relative to ionic substances
because the atoms in a covalent compound have a
strong attraction to the atoms that they share
electrons with, but there is little attraction for the
atoms of other nearby molecules.
10. How does the strength of a covalent bond
relate to bond length?
Generally, as the bond length gets shorter, the
strength of the covalent bond gets stronger.
11. How does a covalent bond differ from an ionic
bond?
In an ionic bond, electrons are transferred from a
metal to a nonmetal. Covalent bonds usually form
between nonmetals and involve the sharing of
electrons.
HW Review Continued
 Nonpolar = electrons are shared equally
 Atoms have similar electronegativities – so they pull
on the electrons equally
 Polar = electrons are not shared equally
 Atoms have different electronegativities – so they pull
on the electrons unequally
 15. Ionic/Nonpolar covalent/polar covalent?

a. Na-F: ionic

b. H-I: polar covalent

c. N-O: nonpolar covalent

d. Al-O: ionic or polar covalent

e. S-O: polar covalent

f. H-H: nonpolar covalent
Open Note Quiz
1. What ionic compound do Ca and Br form?


Write the formula
Write the name in words
2. WRITE A PARAGRAPH USING THIS FORMAT:
 Ionic compounds and covalent compounds are
similar because they both
 (write at least 3 similarities here)
 Ionic compounds and covalent compounds are
different because
 Ionic compounds _______________, but
covalent compounds ____________________.
 (repeat for at least 3 differences)
Agenda
 Catalyst
 HW Review
 Open Note Quiz
 Lewis Dot Structures of Molecules
Lewis Dot Structures
 Lewis Dot Structures show how valence
electrons are arranged in atoms and molecules
 Lewis dot structures for single atoms

1e-
Na

5e-
N
2e-
Mg
6e-
O
3e-
Al
7e-
F
4e-
C
8e-
Ne
Rules for covalent bonding
 Each bond is made up of 2 electrons
 Each atom wants to reach the same configuration as
the noble gases
 Duet rule: Hydrogen wants 2 valence electrons
 Octet rule: All other atoms want 8 valence electrons
 Atoms will share electrons with other atoms so that
both get a full valence shell
Drawing Lewis Dots for
Compounds
 Steps:
 1. Draw Lewis Dot structures for the
individual atoms
 Double check your numbers of valence e




2.
3.
4.
5.
6.
Put central atom in the center
Put other atoms around
Draw 2 dots between each pair
Fill in other dots up to 8 (except for H)
Replace all bonded pairs with lines
CH4
Central atom
H
H C H
H
Practice Lewis Dots
 NH3
Total electrons:NH3
H
H N H
 H2O
N
= 5
H3 = 3 x 1 = 3
8
Practice!
 CF4
 HCl
 CH2Br2
 C2H6
Chemistry
Date: 09/06 & 09/07 2012
NOTES: LEWIS DOT
STRUCTURES PART 2
Double and Triple Bonds
 What happens when the total electrons do
not match up with the Lewis structure?
 Example:
O2
 Draw the Lewis dot structure:
Total valence electrons:
O
O
O2 = 2 x 6 = 12
Single bonded structure uses too many electrons!
Double Bonds
 If the structure has too many dots, remove
lone pairs from two atoms and replace with a
double bond
Lone pairs
O
O
 In a double bond, the atoms share 4 electrons
 Replace the bonded pairs with 2 lines
Double Bonds
 Some atoms can have more than one double
bond
 Example: CO2
Total valence electrons:
O
C
O
C
= 4
O2 = 2 x 6 = 12
16