Plants absorb 1. from the atmosphere and release 2. to the atmosphere. 1.
oxygen
nitrogen
carbon dioxide
2.
water
oxygen
carbon dioxide

when atoms of different types of elements join together, they make molecules called compounds. water consists of compound molecules made up of 2 hydrogen atoms and 1 oxygen atom. this is why it's called h2o. water will always have 2 times the number of hydrogen atoms as oxygen atoms.