The Lewis symbol for an element consists of the chemical symbol with dots placed around it to represent the available valence electrons. The Lewis symbol for the common oxygen atom (O), is represented by the chemical symbol (O) surrounded by 6 dots representing the 6 valence electrons: 2 in the 2s subshell and 4 in the 2p subshell.
To become more stable and achieve a full outer electron shell (the so-called 'octet rule'), oxygen often gains two electrons to fill its outer 2p orbital, becoming an ion (O2-).
Thus, the Lewis symbol for the common ion of oxygen O2- is [O] and two additional dots, representing the two extra gained electrons, and the "-" sign after the 2, represents its charge of -2. So, it is represented as [O]2-.
In this step-by-step solution:
1. We first identified the Lewis symbol for an oxygen atom as having 6 valence electrons.
2. We then acknowledged that to achieve a full outer shell and stability, oxygen often gains two electrons becoming an ion (O2-).
3. Finally, we represented the Lewis symbol for the oxygen ion as [O]2-, to indicate the extra electrons and the -2 charge.