Why is it that elements combine to form compounds?

Explanation:

Almost all the elements combine to form compounds, although the reactivity may vary from element to element. These combinations take place because almost all the elements are unstable. ... By gaining or losing electrons, ionic compounds are produced. Sharing of electrons results in the formation of covalent compounds

beta decay is the answer

acid rain does not usually kill trees directly. instead, it is more likely to weaken the trees by damaging their leaves, limiting the nutrients available to them, or poisoning them with toxic substances slowly released from the soil. the main atmospheric pollutants that affect trees are nitrates and sulphates.