The procedure that can be followed when confronted with the
name of a compound and you wish to write its formula is as
follows:
Identify the symbol of the cation
(first part of the name) and the anion
Identify the valence or charge of
each symbol and place it in parenthesis just above the
symbol
Cations
(Positive Ions)
All Group 1 elements in the Periodic
Table are +1 in compounds.
All Group 2 elements in the Periodic
Table are +2 in compounds.
Transition elements (have a few
charges) will have a Roman Numeral to tell you what
positive charge to use.
silver is 1+, Zinc is 2+ and
Aluminum is 3+
Polyatomic Ions -just use its
charge.
Anions
(Negative Ions)
Group 17 are 1- (will end with
-ide)
Group 16 are 2- (will end with
-ide)
Group 15 are 3- (will
end with -ide)
Polyatomic Ions -just use its
charge. (Generally ends with -ate or -ite,
watch out for hydroxide OH- and cyanide CN-)
3. Balance the total positive and
negative charge on the cation and anion. You ask yourself do
the total positive charge and total negative charge add up
to zero. If the answer is no then we ask how many of each
ion must we have in order to balance charge. We must have
the same number of positive charges as we do of negative
charges. Another way of saying that is that they must add up
to zero.
4. Once you have determined the number
of units of the cation and anion those become the subscripts
which are placed right after the respective symbol.
So for example what is the
formula of Copper (I) Oxide?
Identify the symbols of the
cation and anion
Copper is Cu and Oxide is O
Identify the charge for
each and place above the symbol in parenthesis
For Copper I that would be 1+ and for Oxide
that would be 2-
Balance the positive and
negative charges
Since each Copper is 1+ and each Oxide is 2- then it will
take two Cu+ to balance one oxide with a 2- so that
2(1+) +
1(2-) = 0. The numbers outside the parenthesis become the
subscripts in the formula
Write the formula placing
the subscripts right after the symbol they go with.
Cu2O
Notice that we don't bother to place a subscript 1 after the
Oxide symbol. That is because a subscript one is understood to
be so. If it was zero it wouldn't appear at all in the formula.
Also note that all binary compounds end in "ide"
Let's try another:
What is the formula of Calcium Nitride?
Identify the
symbols of each part of the name
Calcium
symbol is Ca and Nitride symbol is N
Identify the
charge for each
Calcium
belongs from Group 2 which always has a +2 and Nitride will
be a single Nitrogen with a -3 charge
Balance
charge
Since Calcium
is +2 and Nitride is -3 the only way to balance them is to
have three Calcium's and two nitrides
Write the
symbol beginning with the symbol that is first in the name
and include the subscript after each symbol
Ca3N2
Formula
writing with Polyatomic Ions
1.
Identify the
symbol of the cation (first part of the name) and the anion
The symbol for Iron is Fe and the symbol for
Carbonate which is a polyatomic ion is CO3
2.
Identify the
valence or charge of each symbol and place it in parenthesis
just above the symbol
The valence for Iron (III) is 3+ and the
valence for Carbonate is 2-
3. Balance the
total positive and negative charge on the cation and anion.
You ask yourself do the total positive charge and total
negative charge add up to zero. If the answer is no then we
ask how many of each ion must we have in order to balance
charge. We must have the same number of positive charges as
we do of negative charges. Another way of saying that is
that they must add up to zero. Since an Iron (III) has a +3
charge and the Carbonate ion has a 2- then it would take two
Fe3+ units to balance three CO32-
units
4. Once you have
determined the number of units of the cation and anion those
become the subscripts which are placed right after the
respective symbol.
Fe2(CO3)3
Try
These Ionic
Compounds Highlight to reveal the formula
