Topic: Writing Binary Ionic
Formulas
Do Now: An atom of which element has the
greatest attraction for electrons in a
chemical bond?
A. As
B. Ga
C. Ge
D. Se
Empirical Formulas
= smallest whole-number ratio of
elements in compound
• Ionic compounds have empirical
formulas only
Identify the empirical formulas
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H2O Empirical – NOT ionic
H2O2 Molecular – NOT ionic
CaF2 Empirical, Ionic
NaCl Empirical, Ionic
CH4 Empirical – NOT ionic
C2H6 Molecular – NOT ionic
NiO Empirical, Ionic
FeCl3 Empirical, Ionic
Formula Unit
= simplest ratio of ions in ionic compound
Examples:
• KBr is formula unit
– potassium ions & bromide ions are in a 1-to-1 ratio
• Al(NO3)3 is formula unit
– Aluminum & nitrate ions in 1-to-3 ration
Binary Ionic Compounds
• Composed of two different elements
–Positive monatomic metal ion
–Negative monatomic nonmetal ion
Note: A binary compound may contain more than
2 ions but only 2 kinds of ions like Al2O3
Oxidation Number or State
Na+1
• charge of monatomic ion
• has right superscript
Ca+2
O-2
• Some elements form only 1 ion
Al+3 and Ag+1
• Some elements can form more than 1 ion
Fe+2 and Fe+3
Oxidation States
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Group 1 ions always +1
Group 2 ions always +2
MOST group 13 ions are +3
Groups 14, 15, 16, and 17 have multiple
oxidation states
– both (+) and (-)
• Transition metals (Group B) can have
more than 1 oxidation state
– but are always (+)
Writing Formulas for
Binary Ionic Compounds
• first rule in writing formulas for ionic
compounds is
POSITIVE ION FIRST
But how do you get the subscripts?
start by writing formulas from the ions
• Compounds are electrically neutral
– Sum of all charges in compound
should be 0
– (+) and (-) charges must = each other
Equal but Opposite Charges
• Na+1 and Cl-1:
NaCl
+1 + (-1) = 0
• Mg+2 and O-2: MgO
+2 + (-2) = 0
• Al+3 and P-3:
+3 + (-3) = 0
AlP
• Rule: write symbols, (+) FIRST!
Try a few formulas:
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LiI
Li+1 and I-1
Ca+2 and O-2 CaO
Al+3 and N-3
AlN
K+1 and F-1 KF
BaS
Ba+2 and S-2
Criss-Cross Method
• When the charges don’t cancel out
immediately:
• Mg+2 and Cl-1, CROSS and DROP!
(# only, forget signs!)
• Mg1Cl2 but if subscript is 1, forget it!
• MgCl2 means 1 Mg+2 and 2 Cl-1
Check the Math
• MgCl2 means 1 Mg+2 and 2 Cl-1
(1 x +2) + (2 x -1) = +2 + -2 = 0
• charges MUST add up to zero!
Try a few formulas:
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Ca+2 + Cl-1 CaCl2
Na2O
Na+1 + O-2
Cs+1 + S-2 Cs2S
AlCl3
Al+3 + Cl-1
Al+3 + Se-2 Al2Se3
MgF2
Mg+2 + F-1
K+1 + N-3 K3N
Of course, it gets more difficult
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KF
Potassium (K) and Fluorine (F)
Zinc (Zn) and Iodine (I)
ZnI2
Sodium (Na) and Oxygen (O)
Na2O
Magnesium (Mg) and Oxygen (O) MgO
Aluminum (Al) and Oxygen (O)
Al2O3
Calcium (Ca) and Bromine (Br) CaBr2
Cesium (Cs) and Iodine (I)
CsI
Silver (Ag) and Sulfur (S)
Ag2S