In this chapter, we discuss the various ways in which the chemical elements combine to form minerals, but this leads us to ask a broader question: How do these elements form in the first place? Naturally occurring chemical elements are numbered from 1 to 92, depending on the number of protons (or positive charges) in their atomic nuclei. The names we give these elements depend on this number because the number of protons determines an element’s chemical properties.
Given that all protons are positively charged, how can their mutual repulsion be overcome so that as many as 92 protons can be packed into the nucleus of an atom? The answer to this question lies in the heavens, not on Earth. There is compelling evidence that the chemical elements are formed during the life cycle of stars. Because positively charged protons repel each other, it is not surprising that hydrogen, the simplest element, is by far the most abundant element in the universe, and the second simplest, helium is the second most abundant. Together, these two elements comprise 98% of the chemistry of the universe and are believed to have been the only elements present early in its history. The heavier elements have been synthesized progressively as the universe evolved.
The normal tendency for positively charged particles to repel each other can only be overcome at extremely high temperatures where particles travel at very high velocities. For example, helium is formed by the fusion (joining) of two hydrogen protons, a process known as thermonuclear fusion. Thermonuclear fusion reactions occur within the Sun’s core at temperatures of about 15 million oC (or 27 million oF) where hydrogen protons reach velocities of 500 km/sec (or over 1 million miles per hour). This process cannot be duplicated by natural means on Earth.
But how were the elements heavier than helium formed? In stars larger than the Sun, a succession of reactions may occur that can synthesize heavier elements. Helium atoms may fuse first to form carbon, then oxygen and, it is hypothesized, may ultimately form elements as heavy as iron, which has an atomic number of 26 (Fig. 3A). This process depends greatly on a star’s core temperature since the production of heavier elements requires the temperature to be ever higher. Iron is the most tightly packed of all the elements, so no element higher in the periodic table can be formed by thermonuclear fusion.
So how do elements heavier than iron form? Large stars eventually exhaust their fuel and die in supernova explosions like the one shown in Fig. 3B. The unimaginably high pressures produced by such explosions can overcome the mutual repulsion between protons to synthesize chemical elements heavier than iron. Many scientists believe that supernovae are responsible for producing all of the heavier, naturally occurring elements in the universe. Because these elements are synthesized only during such catastrophic events, they are relatively rare.
The explosion of supernovae also disperses the synthesized elements into space where they form the raw materials for future solar systems and become incorporated into other stars and planets. Thus it is likely that the chemical elements on Earth were actually synthesized during the life cycles of unknown and now extinct stars. In a very real way, we are all composed of stardust!
Thermonuclear fusion, by which elements as heavy as iron (atomic number 26) are synthesized in the cores of massive stars, occurs in a succession of steps. Hydrogen atoms first fuse to form helium – the nuclear reaction that powers our own Sun. Helium atoms, in turn, fuse to form carbon, then oxygen and, depending on the star’s core temperature, eventually elements as heavy as iron.
These two photos were taken before and after a supernova explosion. The first image (a) shows the location of the star before the explosion, and the second images shows (b) the flash of light emitted by the supernova explosion. This explosion was photographed in 1987, but actually took place about 170,000 years earlier (it took 170,000 years for the flash of light to reach Earth). Supernova explosions occur in large stars. The energy released by these explosions can synthesize elements with up to 92 protons.