Lesson: Chapter - 13

Formula Writing

The naming is the tricky bit! Once you’ve been given the name, the formula writing is easy as long as you know the formula and charges of the polyatomic ions. The prefixes of a molecular compound make it really easy since the prefix tells you how many atoms are present! Roman numerals are your friend: they tell you the charge on the transition metal.

Remember, Ag, Cd, and Zn are usually not written with a Roman numeral—arrange the symbols in alphabetical order, and the first listed is +1, while the other two are +2. Most important, the sum of the charges must add up to zero in order to form a neutral compound. The “crisscross method” is very useful—the charge on one ion becomes the subscript on the other. Always double-check to see that the subscripts are in their lowest terms! Here are some examples:

Fill in the following chart with the proper chemical formulas. If the charges are missing, it’s because you should already know them or be able to determine them from their position in the periodic table.

Fill in the following chart with the proper chemical names. If the charges are missing, again, it is because you should already know them or be able to determine them from their position in the periodic table.

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