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Oxidation # & Writing
Formulas
Oxidation Numbers
• A positive (+) or negative (-) number
• Shows the number of electrons gained, lost, or
shared when forming a compound
• Sometimes called the “CHARGE” of the atom
• An atom’s oxidation number tells us how many
electrons it needs to fill an octet
I. Oxidation Numbers
d. GAINING electrons results in a NEGATIVE
Oxidation Number. Non-metals gain
electrons
i. Sulfur (Group 16)
1. Gains 2 electrons to reach an octet
2. Becomes S-2
e. LOSING electrons results in a POSITIVE
Oxidation Number. Metals lose electrons.
i. Calcium (Group 2)
1. Loses 2 electrons to reach an octet
2. Becomes Ca+2
g. Oxidation Numbers
+1
+2
+4
or
+3 -4 -3 -2 -1
II. Exchanging Electrons: LITHIUM
a. In Group 1 (the Alkali Metals)
b. Has 1 valence electron
c. To become stable:
i. Gives 1 electron to another atom
d. Now has
i. 3 Protons
ii. 2 Electrons (one more protons (+) than electrons)
iii. Charge of +1
III. Chemical Formulas
a. Molecular Formulas
i. Shows the elements in a compound and the
numbers of each element
b. Subscript
i. Shows the ratio of how many atoms of an element
are present in a compound
ii. Small number at the bottom right of the element
c. Chemical Formula Example: Water
i. H2O
subscript
2 atoms of Hydrogen
1 atom of Oxygen
d. Chemical Formula Example: Glucose
i. Molecular Formula: C6H12O6
ii. 6 atoms of Carbon
iii. 12 atoms of Hydrogen
iv. 6 atoms of Oxygen
IV. Chemical Bonds
a. ION
i. An atom that has a positive or negative charge
resulting from the GAIN or LOSS of electrons
b. IONIZATION
i. The process of becoming an ion
IV. Chemical Bonds
c. CATION
i. A Positive Ion
ii. Metals
iii. CATion = PAWSitive
d. ANION
i. A Negative Ion
ii. Nonmetals
Ca +2
O -2
V. Ionic Bond
a. The FORCE of ATTRACTION between the
OPPOSITE CHARGES of the ions in an
ionic compound (OPPOSITES ATTRACT)
b. ALWAYS form between METALS and
NONMETALS
i. Metals LOSE electrons
ii. Nonmetals GAIN electrons
Ionic Bond & Lewis Dot
i. Can show the transfer of electrons
Na
1+ +
Cl
1-
1+
Na
1-
Cl
VI. Ionic Compounds
a. Have both CATIONS and ANIONS so NET
CHARGE = Zero
b. Example
+1
Na
+
-1
Cl
NaCl
+1 + -1 = 0
c. Properties
i. Dissolve in Water
iii. Strong Bonds (Electrostatic)
d. Which Groups are Most Likely to Bond
Easily
i. Alkali Metals & Halogens (SALT FORMERS)
– Na & Cl
– K & Br
Example: HALITE
ii. Alkaline Earth Metals & the Oxygen Group
– Mg & O
– Ca & S
e. SIDE NOTE: Ionic Compounds &
Conducting Electricity
i. Ionic bonds break down in water, allowing the
cations (+) and anions (-) to move about in the
water
ii. The cations and anions carry the charge
F. Writing Ionic Formulas
• Write the symbol of the element that has the
positive oxidation number (Metals & H)
• Write the symbol of the element that has the
negative oxidation number (Nonmetals)
• Use the CRISS-CROSS Method to make the
sum of the oxidation numbers = zero
• Simplify if there is a common factor
Ionic Compound Example
POTASSIUM
K +1
K +1
SULFUR
S-2
+2
-2
You needed 2 potassiums and 1 sulfur to equal zero
so
K2S
**NOTE: Do not write the Subscript “1”**
Ionic Compound Example
POTASSIUM
K +1
SULFUR
S-2
K2
S1
K2S
**NOTE: Do not write the Subscript “1”**
Ionic Compound Example
MAGNESIUM
Mg +2
Mg1
CHLORINE
Cl-1
Cl2
MgCl2
**NOTE: Do not write the Subscript “1”**
CrissCrossing
H +1
B
+3
C -4
O -2
CrissCrossing
C -4
H +1
O -2
C -4 H +1
O -2
H +1
H2O
H 4C
B +3
C -4 B +3
O -2
B +3
B4C3
B2O3
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