Of all the naturally occurring substances around us every day, there are only 92 that cannot be chemically changed to simpler substances. Two or three of these substances are so rare in nature that their natural occurrence is questionable. There are also 17 other known substances that are man-made in sophisticated instruments like cyclotrons. Together, these 109 pure substances are called elements. The atoms of the 92 naturally occurring elements are the building blocks for all of the substances in the universe.
When atoms join together in various combinations of kind and number, they form molecules. When molecules are made from two or more different kinds of atoms, the substances are called compounds. If molecules are made from only one kind of atom, the substances are elements. New combinations of the atoms produce new molecules and therefore different substances.
Special kinds of chemical formulas, called molecular formulas, are used to represent the kinds of atoms and the number of each kind of atom in a molecule. The simplest molecules are composed of just two atoms (usually a single atom is not referred to as a molecule), which may be the same or different. Oxygen gas (O2), hydrogen gas (H2), and nitrogen gas (N2), are made up of molecules composed of just two atoms of oxygen, hydrogen, or nitrogen respectively. Since these substances are composed of only one kind of atom, they are elements. Carbon monoxide (CO) is a gas with molecules composed of one atom of carbon and one atom of oxygen and carbon dioxide (CO2) is a gas with molecules composed of one atom of carbon and two atoms of oxygen. Water molecules (H2O) are composed of one atom of oxygen and two atoms of hydrogen. These substances are compounds because the molecules that make it up have two kinds of atoms. Many molecules, especially those in living things such as sugar, fat, or protein molecules and molecules of DNA or RNA, are much larger and more complex.
It is possible for all the different substances in the universe to be produced from only 92 naturally occurring elements because there are endless ways to combine these 92 kinds of atoms, which are the building blocks for all molecules. The 26 letters of the alphabet can be compared to the 92 different kinds of atoms. Different words can be formed in many different ways: by using different letters, such as dog and dig and dodge and dug and dugout; or by using the same letters with different arrangements, such as dog and God and good; or mate and tame and meat, or met and meet and teem. Because of all these possibilities, the 26 letters of the alphabet form all the millions of words of the English language. Similarly, new substances form when different kinds or different numbers of atoms join, or when the same kinds of atoms join in different arrangements. And, just like the letters of the alphabet, the 92 naturally occurring elements form all the millions of different substances in the entire universe including all the various metals, plastics, materials for building, fabrics, all parts of all living things, etc. It is the kind, number, and arrangement of atoms within the molecule that determines what the substance is.