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

b. **Steps in Formula Writing. **In writing formulas for compounds, there are four

steps that should be followed.

(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.

c. **Example**. Write the formula for calcium chloride.

(1)

Calcium = Ca, Chloride = Cl.

(2)

Ca valence is +2, Cl valence is -1.

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(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

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must have two Cl ions (-1 x 2 = -2). Thus, the formula would be CaCl2.

MD0803

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