The element hydrogen makes up approximately 70 percent of the sun by mass. Hydrogen is the smallest element, having an atomic number of 1. An element's atomic number describes the number of protons, or positively charged particles, in its nucleus. An element's mass number describes the number of protons and neutrons (uncharged particles) in its nucleus. The most abundant isotope of hydrogen has a mass number of 1. Less abundant isotopes of hydrogen have mass numbers of 2 and 3, respectively. Hydrogen exists as a diatomic gas.
Helium accounts for approximately 28 percent of the sun. Helium is the second smallest element, having an atomic number of 2. The most abundant isotope of helium has a mass number of 4, so it has two protons and two neutrons. Helium is a noble gas, which means that its electron shells are full. As a result, it is not chemically reactive and exists as a monoatomic gas.
Carbon, nitrogen and oxygen account for approximately 1.5 percent of the sun's mass. Carbon has an atomic number 6 and a mass number 12, nitrogen has an atomic number 7 and a mass number 14, while oxygen has an atomic number 8 and a mass number 16. Carbon is the heaviest element made by sun. Fusion reactions that produce larger elements require more heat than exists on the sun. The fact that the sun has elements larger than carbon must therefore mean that they are left over from the sun's predecessor stars.
Other elements make up the remaining 0.5 percent of the sun's mass. The heaviest element made by bigger stars is iron, because the production of heavier elements via fusion reactions involves a net loss of energy rather than a net release of energy. Because iron is too large to be produced on a star as small as the sun, the iron on the sun must be a remnant of the sun's predecessor star. It was once theorized that the sun was simply a cauldron of liquid iron, but this theory did not withstand scientific scrutiny.