Question: What do brackets in an chemical equation mean?
Disambiguated questions and answers: question: What do square brackets in an chemical equation mean for molecules?
What do round brackets in an chemical equation mean?
What do square brackets in an chemical equation mean for ions?
answer: chemical molecular formula
used around the chemical molecular formula
Molecules with multiple functional groups that are the same
used to enclose the ionic formula
Natural Questions answer: chemical molecular formula (from the Wikipedia article "Defining equation (physical chemistry)")