Ionic Compounds: Writing Formulas

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Ionic CompoundsWriting Formulas
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Empirical Formulas

• formulas with smallest whole-number ratio of
  elements in compound
• ionic compounds are ONLY written as empirical
  formulas

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Identify Formulas ? empirical vs molecular ?
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

• simplest ratio of ions in ionic compound
• examples
• KBr is formula unit
• K1 ions Br-1 ions in 1-to-1 ratio
• AlCl3 is formula unit
• Al3 Cl-1 ions in 1-to-3 ratio

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Binary Ionic Compounds

• composed of two different elements
• since ionic composed of
• () monatomic metal ion
• (-) monatomic non-metal ion

note binary compound may contain more than
2 ions but only 2 kinds of ions (ex
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

• A group metal elements
• group 1 ions always 1
• group 2 ions always 2
• MOST group 13 ions are 3
• groups 14, 15, 16 have multiple () oxidation
  states
• B group metal 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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• reminder compounds are electrically neutral
  therefore
• sum of ALL charges in compound
• MUST be equal to zero
• () and (-) charges must each other

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Equal but Opposite Charges

• ions formula
• 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 opposite but NOT equal
• 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 these

• 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)
• aluminum (Al) and oxygen (O)
• silver (Ag) and sulfur (S)

KF
ZnI2
Na2O
Al2O3
Ag2S
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and more challenging

• gold 1 and fluorine
• gold 3 and fluorine
• titanium 2 and oxygen
• titanium 3 and oxygen
• titanium 4 and oxygen
• manganese 4 and bromine
• manganese 7 and bromine

AuF
AuF3
TiO
Ti2O3
Ti2O4
?
Ti O2
MnBr4
MnBr7
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POLYATOMIC IONS

• Table E groups of covalently bonded atoms that
  have a charge () or ()
• polyatomic ions have names
• (-) polyatomic ions can form ionic bonds with ()
  metal ion

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Ternary Compounds

• contain 3 or more different elements
• contain polyatomic ions
• ()metal ions can bond to () polyatomic ions
• () polyatomic ions bond to something (-) charged
  
• either (-) non-metal ion or (-) poly ion

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Formulas with Polyatomics

• Whats the formula for the compound formed from
  Al3 and (SCN)-1?
• charges must add up to zero
• write symbols, positive first!
• figure out how many of each ion needed

Al(SCN)3
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ions with equal but opposite charges are easiest

• Na1 and (OH)-1
• K 1 and (HCO3)-1
• Mg2 and (CO3)-2
• Li 1 and (NO3)-1
• Ca2 and (SO4)-2

Na(OH)

• K(HCO3)

• Mg(CO3)

• Li(NO3)

• Ca(SO4)

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these are more challenging

• Mg2 and (PO4)-3
• Al3 and (NO3)-1
• Fe2 and (OH)-1
• Hg22 and (SCN)-1
• Mg2 and (HCO3)-1
• Al3 and (C2O4)-2

Mg3(PO4)2
Al(NO3)3
Fe(OH)2
Hg2(SCN)2
Mg(HCO3)2
Al2(C2O4)3
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even more challenging

• zinc nitrate ion
• magnesium hydroxide ion
• lithium carbonate ion
• potassium sulfate ion
• calcium phosphate ion
• beryllium chlorate ion

Zn(NO3)2
Mg(OH)2

• Li2(CO3)

• K2(SO4)

Ca3(PO4)2
Be (ClO3)2