How can atoms get a full (maximum energy) outer shell?

Explanation:

One easy way for atoms to achieve a full outer shell is to share electrons in their outside, valence shells. When atoms share their valence electrons, they form a molecule with covalent bonds. One pair of electrons makes one covalent bond.

hydrogen is a compound or a single element and a element is a pure substence that cannot be separated into simpler subtances

clorine is a compound its a chemical substance that forms when two or more different elements combine

so both hydrogen and clorine are compounds

on island x the species needed to adapt because they were being subject to predators which didn't inhabit island y.