Answer
Parentheses must be used in two instances in a formula, both of which have to do with polyatomic ions. First, parentheses must be used when a compound contains more than one polyatomic ion, so that we can distinguish the different ions from each other. Parentheses are also used when we have several ions of one kind of polyatomic ion, and we do this to clearly show that we have several molecules of such ion. Normally when writing subscripts, we do not write parentheses because it's clear which atom is linked to that subscript. With polyatomic ions, however, parentheses are included to show that the subscript corresponds to all the atoms in the polyatomic ion.
Work Step by Step
Parentheses must be used in two instances in a formula, both of which have to do with polyatomic ions. First, parentheses must be used when a compound contains more than one polyatomic ion, so that we can distinguish the different ions from each other. Parentheses are also used when we have several ions of one kind of polyatomic ion, and we do this to clearly show that we have several molecules of such ion. Normally when writing subscripts, we do not write parentheses because it's clear which atom is linked to that subscript. With polyatomic ions, however, parentheses are included to show that the subscript corresponds to all the atoms in the polyatomic ion.
