Ionic Compounds: Writing Formulas

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Ionic CompoundsWriting Formulas
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Empirical Formulas

• formulas have smallest whole-number ratio of
  elements in compound
• Ionic compounds only written as empirical formulas

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Identify ? empirical ? ionic

• H2O
• H2O2
• CaF2
• NaCl
• CH4
• C2H6
• NiO
• FeCl3

Empirical NOT ionic
Molecular NOT ionic
Empirical, Ionic
Empirical, Ionic
Empirical NOT ionic
Molecular NOT ionic
Empirical, Ionic
Empirical, Ionic
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Formula Unit

• is simplest ratio of ions in ionic compound
• Examples
• KBr is formula unit
• potassium ions bromide ions are in a 1-to-1
  ratio
• AlCl3 is formula unit
• Aluminum chloride ions in 1-to-3 ration

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Binary Ionic Compounds

• Composed of two different elements
• positive monatomic metal ion
• negative monatomic non-metal ion

Note A binary compound may contain more than 2
ions but only 2 kinds of ions like Al2O3
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Oxidation Number or State

• charge of monatomic ion
• (right superscript)
• Some elements form only 1 ion
• Some elements can form more than 1 ion

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Oxidation States

• Group A elements
• 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 (-)
• Group B elements (transition metals)
• can have more than 1 oxidation state
• but are always ()

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Writing Formulas for Binary Ionic Compounds

• first rule in writing formulas
• POSITIVE ION FIRST

How get the subscripts?
start by writing formulas from the ions
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• Compounds are electrically neutral
• Sum of all charges in compound
• should be 0
• () and (-) charges must each other

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Equal but Opposite Charges

• Na1 and Cl-1 NaCl 1 (-1) 0
• Mg2 and O-2 MgO 2 (-2) 0
• Al3 and P-3 AlP 3 (-3) 0
• Rule write symbols, () FIRST!

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Try a few formulas

• Li1 and I-1
• Ca2 and O-2
• Al3 and N-3
• K1 and F-1
• Ba2 and S-2

LiI
CaO
AlN
KF
BaS
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Criss-Cross Method

• charges NOT equal but opposite
• Mg2 and Cl-1, CROSS and DROP!
• ( only, forget
  signs!)

• Mg1Cl2 (if subscript is 1, forget it!)

MgCl2
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Check the Math

• MgCl2 means 1 Mg2 and 2 Cl-1
• (1)(2) (2)(-1) 2 -2 0
• charges MUST add up to zero!

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Try a few formulas

• Ca2 Cl-1
• Na1 O-2
• Cs1 S-2
• Al3 Cl-1
• Al3 Se-2
• Mg2 F-1
• K1 N-3

CaCl2
Na2O
Cs2S
AlCl3
Al2Se3
MgF2
K3N
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Of course, it gets more difficult

• Potassium (K) and Fluorine (F)
• Zinc (Zn) and Iodine (I)
• Sodium (Na) and Oxygen (O)
• Magnesium (Mg) and Oxygen (O)
• Aluminum (Al) and Oxygen (O)
• Calcium (Ca) and Bromine (Br)
• Cesium (Cs) and Iodine (I)
• Silver (Ag) and Sulfur (S)

KF
ZnI2
Na2O
MgO
Al2O3
CaBr2
CsI
Ag2S