Why do polyatomic ions form and how are they stabilized?

[UMath1]

I understand that monoatomic ions such as Sodium and Magnesium form to fill there outer shells, but why do polyatomics form? I know that CO2 is a pretty stable compound, so why does carbonate even form? Likewise what leads to the formation of ammonium?

 

[Bystander]

to fill there outer shells

How many ways can monovalent atoms/ions share one electron? How many ways can polyvalent atoms/ions share how ever many electrons?

 

[UMath1]

I know they can meet the octet rule, but why do they occur when the covalent bond is more stable?

 

[DrDu]

In NH4+, bringing the hydrogen nucleus inside the lone pair on N, the electrons gain an enomous amount of potential energy.
You also have to take in mind that most of the ions are only stable in either an ionic crystal or in solution. E.g. discussing the stability of carbonate, you really should consider a reaction like $\mathrm{Na_2O+CO_2 \to Na_2CO_3}$. Besides changes in the electrostatic attraction between the ions (Born Madelung energy), which is difficult to estimate, there is at least one additional covalent bond in carbonate as compared to CO2 and O2-.