a. Formulas. Formulas are combinations of symbols that represent a
compound. A formula indicates which elements are involved and the number of atoms
of each element contained in the compound. In writing formulas, we use subscripts,
coefficients, and parentheses in addition to the symbols of the elements. Subscripts
indicate the number of atoms of an element, as in H2 where two is the subscript
meaning two hydrogen atoms. If there is no subscript with a symbol, it is assumed
there is only one atom of that element. Coefficients, numbers in front of the formula,
indicate the number of molecules of compound, as in 4HCl where four is the coefficient
indicating four molecules of HCl. Parentheses are used to separate a radical from the
rest of the formula when it would be confusing not to do so. In HNO3, it is not necessary
to include parentheses for the NO3 - radical since there is little chance for confusion.
However, we use parentheses for the same radical if it appears NO3 in a compound
such as Hg(NO3)2 where the 2 indicates that we have two NO3 - radicals.
b. Steps in Formula Writing. In writing formulas for compounds, there are four
steps that should be followed.
Determine the symbols for the elements in a compound.
Determine the valence of each of the atoms or radicals.
Write the positive element's symbol first, followed by that of the negative
Make the compound electrically neutral by using subscripts.
c. Example. Write the formula for calcium chloride.
Calcium = Ca, Chloride = Cl.
Ca valence is +2, Cl valence is -1.
(3) Ca+2Cl . If we add the charges, we find that this compound is not
neutral (+2 - 1 = +1). Therefore, we must proceed to step (4).
(4) To have two negative charges to balance the two positive charges, we
must have two Cl ions (-1 x 2 = -2). Thus, the formula would be CaCl2.