Why is the atomic mass for calcium a decimal?

It shows how much atomic mass of calcium weights by its self. You round it so it says 40.08 it would just be 40.

Explanation:

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.

it would be b. 6co2 + 6h2o --> c6h12o6 + 6o2