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I checked the Wikipedia page, but it did not seem to provide a very intuitive explanation.

What distinguishes a molecule such that it behaves like a polyatomic ion? I understand that it is a covalent bond, but what distinguishes it from other covalent bonds that allow it to act like one element. Why does it have a charge, unlike many other compounds (ionic and covalent)?

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A polyatomic ion is simply a molecule that has gained or lost one or more electrons, thus acquiring a net charge. This can occur due to oxidation or reduction of the molecule, by association of an atomic ion with a neutral molecule or by autoionisation of a neutral species into a anion/cation pair.

A concrete example of the latter two processes is the autoionisation of water, which to a first-order approximation may be considered as $\ce{2H_{2}O <=> H_{3}O^{+} + OH^{-}}$. This is coincidentally an example (one of many) of an aqueous reaction involving covalency with a water molecule, addressing in part one of your other questions.

There also exist polyatomic ions known as zwitterions (from German 'zwitter', meaning 'hermaphrodite' or 'hybrid') which possess localised opposite charges on different atoms, but are neutral overall.

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  • $\begingroup$ Thanks. Why are polyatonic ions treated as a singular element though? (as in the case of double displacement, single displacement etc) $\endgroup$ – user612 Sep 11 '12 at 3:34
  • $\begingroup$ @FarhadYusufali - Because for study purposes you have been shown a subset of reactions to study that don't involve the covalent bonds within the ions being broken. Instead, I imagine you are looking at reactions involving ionic bonds being broken in solution. $\endgroup$ – Richard Terrett Sep 11 '12 at 3:50

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