Names and Formulas of Ionic Compounds

Question

Why do these elements in the formulas need to be in parentheses?

Ba3(PO4)2

(NH4)2O

Fe(ClO)3

Answer 1

Because they are present as polyatomic ions: phosphate (PO₄^3-), ammonium (NH₄⁺) and hypochlorite(ClO^-)

These are ionic compounds so, in the first case Barium (2+) and phosphate (3-) come together to form a neutral crystal with proportion 3 to 2.

Answer 2

Why do these elements in the formulas need to be in parentheses?

Ba3(PO4)2

(NH4)2O

Fe(ClO)3

In each case (phosphate PO₄, ammonium NH₄, and hypochlorite ClO), the atoms in parentheses form a strongly bound ion, or charged group, that acts as a single entity. For example, the phosphate ion (PO₄)^-3 carries a triple-negative charge .

The electron dot representation crudely given here shows the 5 electrons (x) of the phosphorous 3s3p shell surrounded by oxygen atoms with 6 electrons (.) each in their 2s29 shell, plus 3 electrons (-) from the barium. This satisfies the octet rule for all five atoms of the group.

.-

: O :

.. ._x ..

: O ^._x P _x^. O .^-

.. ^xx ..

: O :

.-

When Ba3(PO4)2 is dissolved in water, the negative phosphate anions are freed from the positive barium cations by the dispersive action of the highly polar water molecules. The dipole moment of the water is not strong enough, however, to break up the phosphate group into individual atoms. By convention, parentheses are used for such groups of atoms (i.e., ions) rather than inventing another symbol.