Counting Atoms
Name ____________________________________________
The formula for a compound indicates the elements that make up the compound and
the number of atoms of each element in the compound. These numbers of atoms
are indicated by the use of small numbers, called subscripts. Sometimes groups of
atoms act as a single atom—these are called polyatomic ions. If a polyatomic ion is
used in the formula more than once, it is put in parentheses and the subscripts
appear outside the parentheses. When a subscript appears outside the parentheses,
it indicates that ALL the elements inside the parentheses should be multiplied by
that subscript. For example, the formula Fe(OH)3 indicates the combination of one
atom of iron (Fe), three atoms of oxygen (O) and three atoms of hydrogen (H).
In the following examples, list each element in the compound and the number of
atoms of each element present in the compound. If a coefficient is placed in front of
a formula, multiply every element by that number.
Name
Calcium carbonate
Aspirin
Use
Limestone
Pain
reliever
Name
Use
Calcium
dihydrogen
phosphate
Fertilizer
Cellulose
Found in
wood
products
Aluminum
hydroxide
Used to
make other
aluminum
compounds
C9H8O4
Lanthanum
carbonate
Kidney
medicine
Formula
2 CaCO3
Atoms in Formula
Ca = Calcium = 2
C = Carbon = 2
O = Oxygen = 6
Magnesium
hydroxide
Found in
milk of
magnesia
Mg(OH)2
Ammonium
sulfate
Fertilizer
Paradichlorobenzene
Moth
crystals
4 C6H4Cl2
Ferric nitrate (or
iron nitrate)
Used in
etching
metals
Found in
vinegar
2 C2H4O2
Cacodylic acid
Herbicide
Acetic acid
Trinitrotoluene (TNT)
Explosive
C7H5(NO2)3
Formula
2 Ca(H2PO4)2
3 C6H7O2(OH)3
6 Al(OH)3
3 La2(CO3)3
4 (NH4)2SO4
5 Fe(NO3)3
2 (CH3)2AsO2H
Atoms in Formula