105

Unit 1
Elements of Chemical Structure and Inorganic Nomenclature

1-5. FORMULA WRITING

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 -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.  

Steps in Formula Writing. You should follow these four steps when writing formulas for compounds:

  1. Determine the symbols for the elements in a compound.
  2. Determine the valence of each of the atoms or radicals.
  3. Write the positive element's symbol first, followed by that of the negative element.
  4. Make the compound electrically neutral by using subscripts.
Example. Write the formula for calcium chloride.

Step 1:    Determine the symbols for the elements in a compound.

Calcium = Ca, Chloride = Cl.

Step 2:    Determine the valence of each of the atoms or radicals.

Ca valence is +2, Cl valence is -1

Step 3:    Write the positive element's symbol first, followed by that of the negative element.

Ca+2Cl-1. If we add the charges, we find that this compound is not neutral (+2 - 1 = +1). Therefore, we must proceed to Step 4.

Step 1:    Make the compound electrically neutral by using subscripts.

To have two negative charges to balance the two positive charges, we must have two Cl-1 ions (-1 x 2 = -2). Thus, the formula would be CaCl2.

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 SO4 –2  becomes Fe2(SO4)3