[leyown]

I have a question about how polyatomic ions form?

Example: How do we go from H₂O to H₃O+? Wouldn't the oxygen have fulfilled the octet rule? Why would it form another bond with H?

[Schrödinger]

When there are hydrogen ions (H⁺) in the solution, then the lone pair of electrons will attract the H⁺, and oxygen forms a co-ordinate bond with H⁺, thus forming H₃O⁺.
Octet rule isn't violated

[leyown]

I'm trying to picture this with an electron dot structure.



(1) Why do lone pair electrons attract H+ and other ions? Isn't the oxygen stable since it has a filled octet?

(2) The final molecular formula is H3O+. Does the + indicate that an electron has been lost on oxygen in order to form a bond with the H?

[Cherriyan]

1) O is stable and has its octet completed in H₂O but it is one of the highly electronegative atoms. Due to this reason it attracts H⁺ towards itself.
2) The '+' sign is because the electron pair which initially resided on O is now being shared by O and additional H which came in the form of H⁺. O has lost no electron. It's octet is still complete! Remember that the incoming H⁺ is without any electron!