Hi there!
Deep sea divers CANNOT use pure oxygen in their tanks. Pure oxygen is deadly, and can kill them. This would be called Oxygen toxicity
Hope this helps you!
~DL ☆☆☆
Most elements on group 18 are the Noble Gases. They already have a complete last level with 8 electrones. Actually they can form compounds but only on the lab and they will not even last half a second.