Slide 1

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Let’s review
• Oxygen
– Symbol?
– Atomic Number?
– Protons?
– Electrons?
– Electron Distribution?
O
8
8
8
1s2 2s2 2p4
• Oxygen has 6 valence electrons.
Episode 502
Electron Dot Diagrams
• Atom’s symbol surrounded by dots to
represent its valence electrons
• Example
• Oxygen
• 6 valence
electrons
• Lithium
• 1 valence
electron
Episode 502
Problem Set 1
Draw the Electron Dot Diagrams for:
Magnesium
Helium
Krypton
Aluminum
Bromine
Potassium
Nitrogen
Silicon
Strontium
Episode 502
Lewis Structure
• Diagram representing the
arrangement of valence
electrons in a molecule
• Most atoms need 8
valence electrons to
become stable. The
exceptions are H and He
which need 2 valence
electrons to be stable.
Episode 502
Lewis Structure for H2
• 2 electrons
belonging to
both atoms
• Represented
by a dash
between the
symbols
H-H
Shared pair
Episode 502
Lewis Structure for Cl2
• Each Cl atom has 7 valence electrons; giving a
total of 14 valence electrons to work with.
Shared pair
Unshared pair
Electrons belonging to only one atom
Represented by 2 dots
Episode 502
Lewis Structure for HCl
H - Cl
Episode 502
• When more than two atoms bond, you must
determine which is central.
• The central atom is:
– Frequently carbon
– Never hydrogen
– Often atom with the smallest electronegativity
• Lewis structure for
CH3I
• (There are a total of
14 valence electrons
to work with.)
H
H C
I
H
Episode 502
Problem Set 2
NH3
CH2Cl2
CH3OH
(a little tricky)
C6H12
(Hint: The carbon atoms form a ring.)
Episode 502
Lewis structure of ethene, C2H4
• (has total of 12 valence electrons)
H
H
C
C
H
H
Type of bond
Pairs of electrons shared
Single bond
Double bond
Triple bond
1
2
3
Episode 502
Problem Set 3
CO2
HCN
CH2O
C2HCl
Episode 502
Polyatomic Ion
• Covalently bonded groups of ions with a
charge
N = 5 electrons
NO3-1
O = 3 x 6electrons = 18 electrons
-1 = 1 extra electron
Has gained 1 electron to give a total of 24
valence electrons to work with
[
O
N
O
O
-1
]
Episode 502
Problem Set 4
PO4-3
SO4-2
NH4+1
CN-1
Episode 502
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