Answer:
-2
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How many electrons does Oxygen need:
We know that oxygen has an electric configuration of:
2 , 6 [8 electrons]
From the octet rule, we know that all elements try to get to the electronic configuration of the nearest noble gas
In the case of Oxygen, the nearest noble gas is Neon with the electronic configuration:
2 , 8 [10 electrons]
From the above, we can see that oxygen needs 2 more electrons in order to attain the nearest noble gas configuration
What will be the Charge on the ion?:
We know that electrons are negatively charged
Once oxygen gains 2 electrons to attain noble gas configuration, it will have 2 more electrons than it should have
So, the charge on the ion will be -2 since there are 2 extra electrons and both of the electrons contribute a negative charge
Hence, the charge is -2