Question

how are parentheses used in chemical formulas?

They are used to show atoms that are not part of the molecule.

They are used to show which atoms are positively charged.

They are used to show which atoms are most easily removed.

They are used to show how atoms are grouped in the molecule.

Answer 1

Generally, we use parentheses in a chemical formula if we have a compound which contains, for example, more than one of a certain type of ion, which is composed of more than one atom (i.e. polyatomic ions).

Let's take, for example, calcium permanganate. This has a chemical formula of:
\[Ca(MnO _{4})_{2}\]

We know that the compound is made up of the positively charged calcium ion and the negatively charged permanganate ion. The calcium ion is simply an atom with two electrons missing from its neutral version, and so will have a charge of +2. Due to a phenomenon called resonance, the permanganate ion's charge of -1 is shared equally throughout the one manganese and 4 oxygen atoms present in its structure. This is the same in any polyatomic ions (e.g: nitrate ion (NO3)-, carbonate ion (CO3)2-, hydrogencarbonate ion (HCO3)-, phosphate ion (PO3)2-), where the charge is distributed evenly amongst the component atoms.

So, in order to form a neutral compound, we would need two permanganate ions and with one calcium ion. This means that we essentially need two -1 charges, or two of everything which holds this -1 charge (i.e. all of the permangante ion) to balance out the charge on the calcium. That is why we put the subscript number '2' after the full MnO4 in parentheses, as shown above.

So, we really use the parentheses to show groups of atoms, or what are really polyatomic ions, which together as this group hold some fixed charge as part of an ionic compound. Hope that makes sense! :)

Answer 2

http://en.wikipedia.org/wiki/Permanganate