A chemical element is a substance that consists solely of atoms that all have the same numbers of protons in their nuclei, and, unlike chemical compounds, cannot be broken down into simpler substances by any chemical reaction. There are 118 known elements, each distinguished by its number of protons, ranging from hydrogen, which has only one proton, to oganesson, with 118 protons. The first 94 elements occur naturally on Earth, and the remaining 24 are are produced in nuclear reactions. All matter is composed of some combination of one or more of these elements.
Hydrogen is the most abundant element in the known universe, constituting about three quarters of all matter. Helium makes up most of the remainder, with oxygen at a distant third. The most common element on Earth is iron, at about 32 percent. It constitutes about 89 percent of the earth's core, but only about five percent of its crust. The second most common is oxygen at 30 percent, followed by silicon, magnesium, sulfur, nickel, calcium and aluminum. The most abundant element in the earth's crust is oxygen, at about 46 percent, followed by silicon at 28 percent, aluminum at eight percent, and then iron, calcium and sodium.
The most abundant element in the human body is oxygen, accounting for about 65 percent by weight. Carbon is the second-most abundant, at 18 percent. Although the number of hydrogen atoms is greater than that of any other element, the mass (weight) of each hydrogen atom is so much less than that of the other elements that its abundance by mass comes in third at ten percent.