● What is the most plentiful element in the universe? The air we breathe is 21 percent oxygen by volume, the water we drink is nine-tenths oxygen by weight, and our bodies are about two-thirds oxygen. Oxygen is the most abundant element in the earth’s crust, over 49 percent by weight. But when it comes to the universe as a whole, oxygen is far from being the most plentiful. Then is it silicon, the second most plentiful element on earth? Or is it nitrogen or carbon or iron? Well, none of these candidates rank anywhere near the winner, hydrogen. Why, hydrogen makes up about three-quarters of the mass, more than 90 percent of all the atoms in the universe! Next comes helium as the second most abundant. All the rest of the hundred or so elements, including oxygen, comprise a mere one percent of the entire physical universe.