Ionic Compounds
Chemistry Joke
Q: Why do chemists like nitrates
so much?
A: They’re cheaper than day rates!
A Review
 Ion: atom or group of atoms that have a positive or
negative charge
 Ions form when an atom or group of atoms gains or
loses electrons
 METALS lose electrons to form positive CATIONS.
 NONMETALS gain electrons to form negative
ANIONS
Having One
Atom!
Monatomic Ions
 Ions consisting of only 1 atom
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– The charge for ions formed from representative
elements can be determined by looking at the
periodic table
Group 1A metals form cations with a 1+ charge
Group 2A metals form cations with a 2+ charge
Group 3A metals form cations with a 3+ charge
Group 5A nonmetals form anions with a 3- charge
Group 6A nonmetals form anions with a 2- charge
Group 7A nonmetals form anions with a 1- charge
Group 8A elements do not form ions
Having Many
Atoms!
Polyatomic Ions
 Tightly bound groups of atoms that behave as
a unit and carry a charge
 Ex: The 10 you had to memorize.
 1- charge: NO3- NO2- OH- ClO3- C2H3O2CO32SO42SO32 2- charge:
 3- charge:
PO43+
NH
 1+ charge:
4
Naming Monatomic Cations
 The name of a cation is the SAME as the
name of the element!!
 Ex: Na atom forms a sodium cation
 Ex: Li atom forms a lithium cation
 How would you name the following ion:
Al+3
Aluminum ion
Naming Monatomic Anions
 The name of a MONAtomic ion ends in “ide”
Ex: Chlorine atom gains 1 electron to become
the CHLORIDE anion
Ex: Oxygen gains 2 electrons to become an
anion named OXIDE
What would we name the following ion?
S-2 Sulfide ion
Practice:
 What do we call:
– A strontium atom when it loses 2 electrons
Strontium ion
– An iodine atom when it gains 1 electron
Iodide ion
– A phosphorus atom when it gains 3
electrons?
Phosphide ion
Naming Ionic Compounds
 The cation comes 1st and keeps its name.
 Monatomic anions end in “ide”
 Polyatomic cations and anions keep their
names.
K2SO4
LiCl Lithium Chloride
Potassium Sulfate
Magnesium
Fluoride
MgF2
LiNO3 Lithium Nitrate
MgCO3
Be(OH)2 Beryllium Hydroxide
NH4Cl
Ammonium Chloride
Magnesium Carbonate
Writing the Formulas!
 Cations and anions combine in a whole number
ratio that produces a neutral compound.
 The total positive charge of the cations will
equal the total negative charge of the anions.
 For example: 1 Al+3 combines with 3 F-1
(1 x +3) cancels with (3 x -1)
Criss Cross Method
3 Step Process for Writing Ionic Formulas
1. Write the ions with their charges
The charges are known from the periodic
table or memorized for polyatomic ions.
2. Cross the charge numbers (dropping the
+ or -) to form the subscripts. (Subscripts
of 1 are not written.)
3. Simplify the subscripts to the lowest
whole number ratio
Writing Formulas for Ionic Compounds
 Write the formula for magnesium chloride.
1. Write the ions with their charges
The charges are known from the periodic table.
Magnesium is in
Group 2A and
forms a 2+ charge.
Mg2+
Mg
Cl1-
Cl
Chlorine is in
Group 7A and
forms a 1- charge.
2. Cross the charge numbers (dropping the +
or -) to form the subscripts. (Subscripts of 1
are not written.)
MgCl2
3. Simplify the subscripts to the lowest whole
number ratio
Writing Formulas for Ionic Compounds
 Write the formula for beryllium oxide.
1. Write the ions with their charges
The charges are known from the periodic table.
Beryllium is in
Group 2A and
forms a 2+ charge.
Be2+
Be
O2-
O
Oxygen is in Group
6A and forms a 2charge.
2. Cross the charge numbers (dropping the +
or -) to form the subscripts
Be2O2
3. Simplify the subscripts to the lowest whole
number ratio. (If both subscripts are the
same, they can be dropped.) BeO
Writing Formulas for Ionic Compounds
With Polyatomic Ions
 Follow the same 3 steps.
 Just add parentheses around the
polyatomic ion before it gets a subscript
Writing Formulas for Polyatomic Ionic Compounds
 Write the formula for calcium hydroxide.
1. Write the ions with their charges
Calcium is in Group Ca2+
2A and forms a 2+
Ca
charge.
OH1OH
Hydroxide is a
polyatomic ion that
has a 1- charge.
2. Cross the charge numbers
(dropping the + or -) to form the subscripts
3. Subscripts of 1 are not written
4. Simplify the subscripts
CaOH2
There are 2 Hydroxide ions
not just 2 Hydrogen atoms.
Ca(OH)2
5. Remember to add parentheses around the
polyatomic ion if a subscript has been added!
Try Some
Na2CO3
 Sodium Carbonate
 Potassium Hydroxide KOH
 Magnesium Phosphate Mg3(PO4)2
Chemistry Joke
Q: What happened when someone
told a joke to two helium atoms?
A: They went, “He He!!”