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In Science, textbook for class IX, chapter $3$ [1,p.38], it states:

The rules that you have to follow while writing a chemical formula are as follows:

  1. in compounds formed with polyatomic ions, the number of ions present in the compound is indicated by enclosing the formula of ion in a bracket and writing the number of ions outside the bracket. For example, $\ce{Mg(OH)2}$. [...]

Now my book further states that hydroxide ($\ce{OH–}$) and nitrate ($\ce{NO3-}$) are polyatomic ions. And then it says:

Formula of sodium nitrate: $\ce{NaNO3}$

But the formula of calcium hydroxide: $\ce{Ca(OH)2}$

My question is that even though both nitrate and hydroxide are polyatomic, why are we using ( ) in notation for latter in their symbolic representations?

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  • $\begingroup$ Which book is this from? $\endgroup$ – Safdar Aug 7 at 15:27
  • $\begingroup$ ncert.nic.in/textbook/pdf/iesc103.pdf - please look at pg 29 and also google search says so - but I could not get the reason why () are used for later and not for former? $\endgroup$ – Programmer Aug 7 at 15:31
  • $\begingroup$ Well, simply put if you wish to write it as so, you would have the compounds to be $\ce{NaNO3}$(sodium nitrate) and Calcium hydroxide as $\ce{CaOH2}$ instead of $\ce{Ca(OH)2}$. The former having 2 hydrogens and one oxygen, wherein reality, it has two oxygens and two hydrogens. $\endgroup$ – Safdar Aug 7 at 15:40
  • $\begingroup$ So CaOH2 instead of Ca(OH)2 are both same? So it is not important to have the brackets - google says Ca(OH)2 or CaH2O2 are same? $\endgroup$ – Programmer Aug 7 at 15:57
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    $\begingroup$ @MaxW They are ( ), it is just that the font makes it look different. [ ] is used for co-ordination complexes. $\endgroup$ – Safdar Aug 7 at 16:57
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What they are trying to say is that hydroxide $\ce{OH^−}$ and nitrate $\ce{NO3^−}$ ions, which are polyatomic anions react bond with cations as a group.

When $\ce{NO3^−}$ bonds to $\ce{Na^+}$ only a single nitrate anion bonds to a single sodium cation, to form $\ce{NaNO_3}$ so brackets are not needed. You can still use them, in which case it would be $\ce{NaNO_3}$ but it is redundant and current convention doesn't use it.

However, when $\ce{OH^−}$ bonds to $\ce{Ca^{2+}}$ two hydroxide anions bond to a single calcium cation, to form $\ce{Ca(OH)_2}$ so brackets are needed to avoid confusion and show the structure of the compound.

Old ways of writing it (before my time) before the conventions were standardised, such as $\ce{CaO2H2}$ or $\ce{CaH2O2}$ don't immediately show that it is a hydroxide, so would have been confusing to non-experts.

To reiterate: the brackets are used when there is more than one polyatomic ion in a compound. So if you write down sodium hydroxide it is $\ce{NaOH}$ but calcium hydroxide is $\ce{Ca(OH)2}$

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There is a direct analogy in mathematics. We write $2\cdot(a+b)$, but we do not write $1\cdot (a+b)$. We write just $a + b$.

So we do not write $\ce{Na(NO3)1}$, but just $\ce{NaNO3}$, as the meaning is obvious even without the explicit parentheses. Sometimes, the parentheses are used with implicit index 1, if it supports the structural clarity of complexes with mixed ligands, like e.g. $\ce{[Fe(H2O)5(OH)]^2+}$ or $\ce{[Fe(CN)5(NO)]^2-}$

OTOH, as there are 2 $\ce{OH}$ groups in calcium hydroxide, we writ0e $\ce{Ca(OH)2}$.

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