Fill in the blank As we look around space we find that most of it is composed of (4) of hydrogen. When the hydrogen combines with carbon it forms a (5), which is the basis for all living things. Combining different (6) together form different substances. A group of two or more atoms that are bonded together is called a (7). And when the elements are combined in a specific way, or ratio, the substance is called a (8) (e. g. water).