Counting Atoms
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 / Use / Formula / Atoms in FormulaCalcium carbonate / Limestone / 2 CaCO3 / Ca = Calcium = 2
C = Carbon = 2
O = Oxygen = 6
Aspirin / Pain reliever / C9H8O4
Magnesium hydroxide / Found in milk of magnesia / Mg(OH)2
Paradichlorobenzene / Moth crystals / 4 C6H4Cl2
Acetic acid / Found in vinegar / 2 C2H4O2
Trinitrotoluene (TNT) / Explosive / C7H5(NO2)3
Name ______
Name / Use / Formula / Atoms in FormulaCalcium dihydrogen phosphate / Fertilizer / 2 Ca(H2PO4)2
Cellulose / Found in wood products / 3 C6H7O2(OH)3
Aluminum hydroxide / Used to make other aluminum compounds / 6 Al(OH)3
Lanthanum carbonate / Kidney medicine / 3 La2(CO3)3
Ammonium sulfate / Fertilizer / 4 (NH4)2SO4
Ferric nitrate (or iron nitrate) / Used in etching metals / 5 Fe(NO3)3
Cacodylic acid / Herbicide / 2 (CH3)2AsO2H