How do you complete and what is it used for?

Use afbau rules to fill in electrons for the Ununoctium(118) element. This is called electron configuration, so you ll have to fallow 3 rules:
- first electrons with same spin. 
- first pair the eletrons in the same orbital (every level/row of dots)
- 3d > 4s... there is crossings you will notice, meaning 3d is an orbital level with higher energy in compare to 4s. So it is written 4s3d4p insetead of 3d4s4p ...

You complete it by adding electrons following example:

2p3 woud be: (|)(|)(|),  and
2p6 would be: (||)(||)(||) 

So in this questionfor Uuo you just have to fill every dot with two straight lines representing electrons... 

The density of water is 1 g/cm cubed and in order for the pebble to sink its density has to be greater than the density of water and in order for the twig to float its density has to be less than the density of water