The Formation of the Periodic Table of Elements
In order for the elementary particles that compose this matter residue to interact with each other to form individual elements, they must be organized into some system. While the early Universe did create helium due to the fusion of hydrogen, more complex elements beyond helium were unable to form as the Universe cooled too fast for the formation of carbon from helium fusion. Nevertheless, the formation of helium was extremely important as that prevented the decay of the neutrons.
We know from stellar evolution that light elements fuse to form heavier ones inside of stars. This thermonuclear fusion process is the way that stars derive their energy and again is a consequence of E = mc2. However, before this process can even begin, stars must form. As star formation proceeds from the gravitational collapse of Giant Molecular Clouds, it is necessary for a gravitational potential well to form in order to collect gas that is destined to fragment into stars. Thus, galaxies serve as gigantic collection places where gas is held together by the large mass which defines the galaxy (most of this mass could be dark matter ). Hence, although the details of galaxy formation remain largely unknown, it's clear that if no galaxies would have formed, there would have been no places in the Universe for gas to collect to form stars and hence the heavy elements that compose planets and people. In this case the Universe would consist only of an expanding sea of hydrogen and helium mixed in amongst all the photons. Moreover, this process may in fact depend on the existence of dark matter as without dark matter, galaxy formation might not have occurred.
Through a complicated process, these massive stars become Supernova. During the actual supernova process, the elements in the star's envelope (which is in the process of being violently ejected) is subject to a flood of neutrons and helium nuclei. These particles are captured from the iron seed nuclei that are present to build up the periodic table. There are two basic mechanisms of neutron capture. These are called the slow and rapid process. During the slow, or S-process, neutron capture proceeds at a sufficiently slow rate that when an element reaches an unstable isotope (adding neutrons to a heavy atomic nucleus creates an isotope) it is allowed to decay into a more stable form before it captures another neutron. Note that when a neutron decays into a proton, the atomic nucleus gains a proton and hence becomes a new element. During the rapid, or R-process, the neutron capture rate is so high, that intermediate decays generally can not occur. We can illustrate the difference between the S-process and R-process by considering how cobalt can be formed from an iron nucleus seed. This is shown in Figure 6.2.
The right hand side illustrates the slow process. In this process the original 56Iron nucleus (which contains 28 protons and 28 neutrons) captures three extra neutrons until 59Iron (28 protons and 31 neutrons) is formed. 59Iron is unstable and before it can capture another neutron, one of the extra neutrons decays into a proton, leaving an element with 29 protons and 30 neutrons. This element is 59Cobalt.
The left hand side illustrates the rapid process. In this case, 59Iron captures another neutron before it can decay and then another one to reach 61Iron (28 protons and 33 neutrons). 61Iron, however, is extremely unstable and quickly decays into 61Cobalt (29 protons and 32 neutrons). In this way, the R-process produces neutron-rich heavy nuclei compared to the S-process . Most of the cobalt on the earth is 59Cobalt, indicating a preponderance of S-process heavy nuclei. However, we know that some R-process material is present and it's been quite important for the evolution of the Earth. In particular, Uranium comes in two forms: 235U and 238U. 235U forms from the S-process and is the principal fuel used in nuclear fission reactors. 238U, however, forms from the R-process . Its abundance on Earth is important as it has a half-life of 4.5 billion years and provides the bulk of the radiogenic heat that is the driving mechanism behind plate tectonics.
Neutron Capture: Two processes:
In the solar system (meteorites) most heavy elements are proton-rich indicating S-processed elements, but some R-processing occurred which is of very important consequence to the earth.
But R-processing produced a small bit of Uranium which is extremely important.
Overall Composition of the Earth:
Composition of the Earth's Crust:
Through the mechanism of Supernova, the Periodic Table of Elements is created. The most miraculous feature of these Supernova is that they represent very local sites of heavy element formation in a galaxy, but when they explode they hurl this heavy element enriched debris out to radii several hundred light years from the point of origin. Thus, Supernova disperse the elements out of which rocky planets form throughout the galaxy. Indeed, it is not an accident that the age of the Solar System is only about one-half the age of the Galaxy as many generations of Supernova must occur in order for there to be sufficient heavy elements in the Interstellar Medium that facilitate the formation of rocky planets. Since this seeding mechanism occurs on a galaxy-wide basis, there is every expectation of many rocky planets in orbit about the stars. The probability of life forming on those distant worlds is discussed later in this course.