Example Question #2 : Sat Subject Test In Chemistry
What is the formula for the compound formed from calcium and sulfate ions?
In order to answer this question correctly, it is important to have knowledge of the formation of ionic bonds, polyatomic ions, and the periodic table. First, it is important to notice that calcium ions and sulfate ions will form an ionic bond. Knowing this, it can be understood that valance electrons will be exchanged between the ions and therefore, oppositely charged ions that are attracted to each other will be formed.
Calcium is in the second column of the periodic table, so it will provide two ions to the sulfate polyatomic ion. This will form a calcium cation () and a sulfate polyatomic anion ().
One calcium ion provides two electrons, which are both accepted by a sulfate polyatomic ion, so the ratio of calcium ions used to sulfate polyatomic ions used is 1:1. This means that only one of each ion is needed to form the compound, making the formula .
Initially it may be tempting to write the formula as ; however, since both ions have a "2" subscript in this form, it is redundant to the formula in this way. Only one of each ion is needed to form the compound.