Q/ A
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How would a compound form when Lithium is
combined with Phosphorous? How many atoms
of each would their be ? How would we write
this out at a compound ?
P -3 valence, Li +1 valence
It would take 3 Li to complete the outer shell of
P.
Li3P
Writing Formulas for
Ionic Compounds
5.8
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What is the formula for the ionic compound
formed by calcium and iodine?
Step 1- write the symbol of the metal or
positively charged ion first. The symbol of the
nonmetal or negatively charged ion is written
second.
Ca
I
Step 2- Write the charge of each element above each
symbol.
2+
1Ca
I
 Step 3- Determine how many ions of each type you
need so that the total ionic charge is zero.
2+
1Ca
I
One Ca2+ ion will balance the charge of TWO I- ions.
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Step 4- write the formula using subscripts to
indicate the number of ions of each type.
CaI2
Crisscross rule
First you need to write the ionic charges above
their symbols.
 Ex:
3+
2Al
S
Then you need to ‘crisscross’ the charges, using
them now as subscripts.
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Ex:

3+
2Al
S
Now that we’ve crisscrossed the charges, add the
subscripts.
Al2 S3
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Crisscross Modified
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The following is an example of when we have to
modify the crisscross rule.
What is the formula for the ionic compound
formed by nickel and oxygen ?
First, write the symbols.
Ex:
Ni
O
Next write the ionic charges above the symbols,
and crisscross them.
 Ex:
2+
2Ni
O
Using the Crisscross rule, you should get Ni2O2.
But why don’t we write this compound this way?
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Our formulas have to have the lowest number
of ions that will produce an electrically neutral
compound. So we divide by the common factor,
in this case 2.
So, the formula for Ni and O = NiO
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Now try it the crisscross rule for this
compound.

Sn combining with O.

+4
Sn
-2
O

Sn2 O4
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But what can we do ?
Divide by the common factor 2.
Making the compound: SnO2
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Questions
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1) a) How do metals form ionic compounds
with nonmetals? B) Describe this process with
an example.
2) Beryllium and fluorine react to form an ionic
compound. A) Which element is the metal and
which is the nonmetal? B) How many electrons
must each element gain or lose to form stable
ions? C) Draw a lewis dot diagram of this
compound. D) Indicate the ionic charges on the
ions. E) What is the overall charge on the
compound? F) What is the chemical formula?
Questions from pg. 189 1-4
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1 a) They form by the metal giving away valence
electrons, and the non-metal taking the valence
electrons.
1b) NaCl- Na gives one, Cl takes one.
2 a) Be = metal, F= non-metal.
B) Be= lose 2, F= gain one
 D) Be= +2, F= -1
 E) Neutral
 BeF2
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Brain Pop
http://www.brainpop.com/science/matterandchemistry/chemicalequations/
Q/A

Sodium and Nitrogen are combining to make a
compound. Write it first as a balanced chemical
compound. Second, name the compound.
1+
Na
3N
Use the crisscross rule:
Na3N
Naming the compound:
Sodium Nitride
Writing Compounds
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When writing out the name of Ionic
Compounds you have to:
Write the name of the metal first- always as it
appears on the periodic table.
Write the name of the non-metal second, but
add the ending ‘ide’.
Ex: CaI = Calcium Iodide
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Name of Element Symbol
Fluorine
F
Chlorine
Cl
Bromine
Br
Oxygen
O
Sulfur
S
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Nitrogen
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N
Charge Name of Compound
1Fluoride
1Chloride
1Bromide
2Oxide
2Sulfide
3-
Nitride
More than One Ionic Charge
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Some metals can have two completely different
ionic charges.
Ex: Copper.
Copper in white form = 1+ charge

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Compound= CuCl
Copper in yellow form= 2+ charge

Compound= CuCl2
So…. Add roman numerals
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When you have elements that can make more than one
ion, you need to show which ion they are forming with
roman numerals.
Ex. CuCl = Copper (I) Chloride
Ex. CuCl2 = Copper (II) Chloride
You will be able to know which roman numeral to use
by the neutral compound’s subscripts.
ONLY USE THIS WHEN THE ELEMENT CAN
HAVE MORE THAN ONE IONIC CHARGE!
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Name of Element Symbol Ionic Charges Roman Numeral

Copper
Iron
Lead
Tin
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Cu
Fe
Pb
Sn
1+, 2+
2+, 3+
2+, 4+
2+, 4+
I, II
II, III
II, IV
II, IV
Changing from Words to Chemical
Symbols
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The last thing we need to understand is how to
change compound names back into chemical
formulas.
Ex: Magnesium Oxide
What do you think we do first ?
Magnesium= metal. Find it on the periodic
table. Write it down with the charge.
2+
Mg
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What do we need to do second?
Oxide= Oxygen, a non-metal. Find it on the
periodic table and write down the charge.
2O
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Now put them together, and complete the criss
cross rule.
2+
2Mg
O
Mg2 O2
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There is a common factor, so we can simplify it
to: MgO
Q/A

Change the name compound of Sodium Iodide,
into a chemical formula.

Na =+1
I = -1
NaI
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Handout
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Part A
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From the chemical formula, write the name
Part B

From the name, write the chemical formula