Module 4 Lesson 2
Ionic Naming and Formulas
Formulas
• Chemical formula
– Shows kinds and numbers of atoms in smallest representative unit
• NaCl, H2O
• Formula unit
– Ionic compounds form CRYSTALS that vary in size, because the ions
attract from all sides, the ions pack together in an alternating pattern,
so the formula shows the lowest whole number ratio of ions
Oxidation Numbers
• Oxidation numbers are the charges that an atom takes on to obey the
Octet Rule
• The oxidation number of an element depends on the Group number of the
element.
Group Number
Oxidation Number
1
+1
2
+2
3-12
Usually +2 except Ag and Zn
13
+3
14
+4
15
-3
16
-2
17
-1
18
0
Mark your Periodic Table
+1
+2
All the transition metals
except Ag and Zn can have
more than one charge and
need a Roman numeral (Pb
and Sn need one too!)
+4
+3
+2
+1
0
-2
-3
-1
Formulas
• Practice
Al + O
Be + S Al + Br
K+I
Mg + I
Na + O
• Sometimes one loses more e- than the other needs, or vice versa, so the
ratio cannot be 1:1
• look at the charges, criss-cross them so that they become the subscripts,
then reduce if possible; the idea is that the overall charge in a chemical
formula is ZERO
• Al+3 O-2  Al2O3
• Be+2 S-2  Be2S2  BeS
• Try the rest of the practice at the top of the page and then check yourself
on the following slide
Formulas and Naming
• Practice
•
Al + O
Be + S Al + Br
K+I
Mg + I
Na + O
Aluminum bromide
• AlBr3
• KI
• MgI2
Potassium iodide
Magnesium iodide
Sodium oxide
• Na2O
• Naming these are easy, you just say the name of the cation and the name
of the anion (ending is changed to –ide)
• Writing these names and formulas is easy because each ion always has the
same charge, but this isn’t always the case
Naming Ionic Compounds
• Remember, ionic compounds always have a metal
• Cations have the same name as the metal (add “ion”)
• Anions have the ending changed to –ide, or they are polyatomic and
have a special name
• Metals that can form more than one type of ion use a Roman numeral
equal to the charge to designate the type
• Fe+2 is iron(II)
• Fe+3 is iron(III)
Variable Oxidation States
• As stated on this previous slide, some elements can have more than one
oxidation state.
• This includes many of the Transition Elements as well as Sn and Pb.
• These require Roman Numerals to show the oxidation state.
For example Pb can be +2 or +4 and would thus be represented:
Pb (II) and Pb (IV)
• These Roman Numerals are also required to name these compounds.
– For example
Lead (II) chloride
– PbCl2
– PbCl4
Lead (IV) chloride
Polyatomic Ions
From the …
Reference Tables
Writing Formulas
• Polyatomic ion: a group of covalently bonded atoms with an overall charge
• Criss cross as you did for the other ionic formulas placing the superscript
form one as the subscript of the other.
• K+ PO4-3
K3PO4
• Mg+2 NO3Mg(NO3)2
• Note that you must add parentheses if the number goes for the entire
polyatomic ion
• Try the following:
– Ca + hydroxide
– K + sulfate
– Al + carbonate
– Ammonium + O
Writing Formulas
• Ca + Hydroxide
Ca+2 OH-
Ca(OH)2
• K + Sulfate
K+ SO4-2
K2SO4
Al + carbonate
Al+3 CO3-2
Al2(CO3)3
Ammonium + O
NH4+ O-2
(NH4)2O
Naming Polyatomics
• Naming ionic compounds containing polyatomics is simple!
• Name as usual except just use the name of the polyatomic ion when it
occurs.
• Example:
• Ca(OH)2 Name the cation (Calcium) and then name the polyatomic ion
(hydroxide)
– Calcium hydroxide
– Go to the next slide to see more examples
Naming Ionic Compounds
• Ca + Hydroxide
Ca+2 OH• K + Sulfate
K+ SO4-2
Ca(OH)2
Calcium
hydroxide
K2SO4
Potassium
sulfate
Al + carbonate
Al+3 CO3-2
Al2(CO3)3
Aluminum
carbonate
Ammonium + O
NH4+ O-2
(NH4)2O
Ammonium
oxide