The full question reads:
“When writing the overall formula of ionic compound involving polyatomic ions, why are parentheses used around a formula of a polyatomic ion when more than one such ion is present? Give an example”
(#143 on take home test)
Consider the analogy that the parentheses are ropes - keeping polyatomics together - keeping them as a unit. Now, consider that the loss or gain of an electron occurs within the unit and not a particular element in the polyatomic.
EXAMPLE
Iron(III) sulfate
Iron(III) = Fe3+
sulfate = SO42-
Now put them together; use the crisscross method.
Fe2(SO4)3
The two next to the Fe means there are two iron(III) cations in the compound. The 3 next to the SO4 mean there are three of the sulfates (kept together with the "rope.")
Consider the analogy that the parentheses are ropes - keeping polyatomics together - keeping them as a unit. Now, consider that the loss or gain of an electron occurs within the unit and not a particular element in the polyatomic.
EXAMPLE
Iron(III) sulfate
Iron(III) = Fe3+
sulfate = SO42-
Now put them together; use the crisscross method.
Fe2(SO4)3
The two next to the Fe means there are two iron(III) cations in the compound. The 3 next to the SO4 mean there are three of the sulfates (kept together with the "rope.")
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