Math Skills
Writing Formulas for Ionic Compounds
What is the formula for the ionic compound calcium chloride?
Read and Understand
What information are you given?
The name of the compound is calcium chloride.
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Plan and Solve
List the symbols and charges for the cation and anion.
Ca with a charge of 2+ and Cl with a charge of 1−
Determine the ratio of ions in the compound.
It takes two 1 − charges to balance the 2+ charge. There will be two chloride ions for each calcium ion.
Write the formula for calcium chloride.
CaCl2
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Look Back and Check
Is your answer reasonable?
Each calcium atom loses two electrons and each chlorine atom gains one electron. So there should be a 1-to-2 ratio of calcium ions to chloride ions.
Math Practice
Write the formula for the compound calcium oxide.
Write the formula for the compound copper(I) sulfide.
Write the formula for the compound sodium sulfate.
What is the name of the compound whose formula is NaOH?
Describing Molecular Compounds
Like ionic compounds, molecular compounds have names that identify specific compounds, and formulas that match those names. With molecular compounds, the focus is on the composition of molecules. 
The name and formula of a molecular compound describe the type and number of atoms in a molecule of the compound.
Naming Molecular Compounds
The general rule is that the most metallic element appears first in the name. These elements are farther to the left in the periodic table. If both elements are in the same group, the more metallic element is closer to the bottom of the group. The name of the second element is changed to end in the suffix -ide, as in carbon dioxide.
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