Writing Formulas for Inorganic Molecules

The description given earlier is for writing molecular formulas for organic molecules. What about finding the molecular formula for the compound shown below? It is not an organic compound, because the molecule contains no carbon atoms. Thus it must be an inorganic compound.

Phosphorus trifluoride:

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Unlike organic molecules, where element symbols are written in alphabetical order, the order of symbols in the molecular formula for an inorganic molecule depends on the positions of its elements on the periodic table. The further left an element is on the periodic table, the further left its symbol is placed in the formula. In phosphorus trifluoride, phosphorus will be listed first since it is in Group VA while fluorine is to its right in Group VIIA. The molecular formula for this compound is PF₃.

Note: If you would like more information on the various elements, click on the periodic table. Then to return to this page you will need to click on the Back icon at the upper left corner of the screen.

The rules given for the organic and inorganic molecules do not include finding formulas for ionic compounds. Ionic compounds do not consist of molecules, and so the rules are different. Please refer to Sections 4.3 and 4.4 in The Chemical World for this material.