Writing Chemical Formulas for Ionic Compounds
Atoms or groups of atoms that gain or lose electrons form charged particles called ions.
Losing electrons results in positive ions (called _________) while gaining electrons
results in negative ions (called __________). When ions combine they form neutral
compounds. In this exercise you will make models of ions and use them to discover how
the formulas for ionic compounds are derived.
Color (or highlight) all of the cations pink. Color (or highlight) all of the anions blue. Cut
out all the cards.
Ionic compounds are composed of two or more charged particles and have no net charge.
Put a Na+ ion card on the desk and place a Cl- so that the minus sign is next to the plus
sign. Notice that there are no extra charges left over. If there were, you would need to add
more cards of the same two ions until all the signs are completely matched.
Now you are ready to write the formula. By convention the positive ion is always written
first. Since there is one Na+ and one Cl- the formula would be Na1Cl1. Subscripts show
how many ions (cards) are in the formula. When there is only one ion, the subscript is
omitted. The correct formula is NaCl. The name of this compound is a combination of
both ion names (in order): sodium chloride.
Use the cards you have made to represent compounds of the following ions:
cation
anion
Chemical formula Chemical name
+
Sodium
Na
Chloride
Cl
NaCl
Sodium chloride
potassium
bromide
calcium
chloride
aluminum
bromide
ammonium
chloride
STOP. Have your teacher check your work.
Ions like nitrate (NO3-) are composed of more than one element. Put together the calcium
and nitrate ion cards to represent an ionic compound with no net charge. You should have
one calcium card and two nitrate cards. To write the formula, use parentheses around the
nitrate symbol and a subscript of two outside the parentheses to show that there are two
nitrate ions (cards). The correct formula would be Ca(NO3)2. The correct name of this
compound would be calcium nitrate.
Use the cards you have made to represent compounds of the following ions:
cation
anion
Chemical formula Chemical name
+2
-1
Calcium
Ca
Nitrate
NO3
Ca(NO3)2
Calcium nitrate
sodium
sulfate
ammonium
phosphate
aluminum
sulfate
calcium
hydroxide
STOP. Have your teacher check your work.
cation
silver
anion
nitrate
barium
chlorate
iron(III)
oxide
nickel
phosphate
Chemical formula
Chemical name
STOP. Have your teacher check your work.
Your ion charts show many of the common ions. Use this information to write chemical
formulas and complete YT p57-58 below.
Mg Ca
Ca
Ca
Ni
Magnesium
Calcium
Calcium
Calcium
Nickel
Ni
Ni
K
Ag
Ba
Nickel
Nickel
Potassium
Silver
Barium
Fe
Fe
Al
Al
Na
Iron
Iron
Aluminum
Aluminum
Sodium
Na
NH4 NH4 NH4 HCO3
Sodium
Ammonium
Ammonium
CO3
Cl
Cl
Carbonate
Chloride
Chloride
O
Ammonium
Bicarbonate
Cl ClO3
Chloride
Chlorate
O
O OH
OH
Oxide
Oxide
Oxide
Hydroxide
Br
Br
Bromide
Bromide
Hydroxide
Br NO3 NO3
Nitrate
Nitrate
PO4 PO4 SO4 SO4
SO4
Phosphate
Phosphate
Bromide
Sulfate
Sulfate
Sulfate