Question regarding electron cloud modle

Jun 2017
1
0
Sri Lanka
Hi,
Nitrate, Carbonate ions have resonance structures. So, they have delocalized pi electron clouds. We know that nitrate and carbonate ions have more than one lewis structure, but resonance structure is the most suitable one.

My problem is this. How that the resonance structure is exist.
*Are they always change from one structure to the other every time.
or
*have an only structure which have the every feature of above structures.