What do you mean by energy conservation

The cannot be destroyed. However, it can be converted into some other forms. This is called inter conversion of energy. For example, rub your hands with one another. In rubbing your hands you have used your muscular energy which then converted into heat energy.

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.