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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 compound. In writing formulas, we use subscripts, the 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 NO_{3} radical since there is little chance for confusion. However, we use parentheses for the same radical if it appears NO_{3} in a compound such as Hg(NO_{3})_{2} where the 2 indicates that we have two NO_{3} radicals.
Steps in Formula Writing. You should follow these four steps when writing formulas for compounds:
Example. Write the
formula for calcium chloride.

Rule of Crossing Valences. A convenient rule for determining what subscripts are necessary in writing formulas is the rule of crossing valences. This rule states that one can take the valence of the element at the left and make it the subscript of the element at the right, and in like manner take the valence of the element at the right and make it the subscript of the element at the left. For example:
Fe^{+3} SO_{4} ^{–2} becomes Fe_{2}(SO_{4})_{3}