The First Elements
The first three elements are hydrogen, helium, and lithium, in order of increasing weight and complexity. Their nuclei have one, two, and three protons, respectively. Their international chemical symbols are H, He, and Li.
There are three types of hydrogen, called (ordinary) hydrogen, deuterium, and tritium, in order of increasing mass and decreasing abundance. The nucleus of ordinary hydrogen consists of a single proton. Deuterium has a neutron as well as a proton. Tritium has two neutrons with its proton.
There are three types of hydrogen, called (ordinary) hydrogen, deuterium, and tritium, in order of increasing mass and decreasing abundance. The nucleus of ordinary hydrogen consists of a single proton. Deuterium has a neutron as well as a proton. Tritium has two neutrons with its proton.
We indicate the total number of nucleons (protons and neutrons) with a superscript number following the chemical symbol. In this notation, the three types of hydrogen are H¹, H², and H³. One should not confuse the superscripts with footnote numbers or with the subscripts chemists use to show the number of atoms in a chemical compound. Probably the best-known chemical compound formula is that of water, H2O, made up of two hydrogen atoms and one oxygen atom. This formula represents ordinary water. If one or the other or both hydrogen atoms is deuterium or tritium one has “heavy water.” The possible forms of heavy water made from heavy hydrogen would be H²H¹O, H²H²O, H³H¹O, H³H²O, and H³H³O. Note that for H²H²O we can write H²2O. Similarly, H³H³O is also H³2O. We usually suppress the superscript 1 for ordinary hydrogen, so H²H¹O is H²HO and H³H¹O is H³HO. None of these formulas is very usual because we have another simplification. Deuterium and tritium are so important that they have their own chemical symbols, D and T, even though chemically they are the same as hydrogen. Thus, we usually symbolize the different kinds of heavy water with DHO, D2O, THO, TDO, and T2O.
The excess of neutrons makes tritium unstable. One of the neutrons is likely to turn into a proton and emit an electron and a neutrino. This turns tritium into the lightweight stable isotope of helium called “helium-three”, He3. Ordinary helium, usually written as He without a superscript, has two neutrons. Lithium-six, Li6, has 3 neutrons but the much more abundant lithium-seven, Li7, has 4 neutrons.
Thus, we have elements with 1, 2, 3, 4, 6, 7, and 9 nucleons, but no elements with 5 or 8 nucleons. This prevents the formation of other elements heavier than beryllium.
These elements formed when the original protons and neutrons collided and stuck together under the intense heat and pressure of early morning on the first day, the first few minutes of the universe. Some of the protons and neutrons made combinations of two, three, four, six, or seven nucleons, while other protons remained free. No free neutrons remained long after the first 15 minutes, because the weak force broke them up into protons, electrons, and neutrinos.
It was unlikely even under those early conditions for three particles to collide simultaneously. The first compound nucleus was deuterium, formed when a neutron hit a proton and the two nucleons stuck together. Almost all the deuterium that exists formed in the first four minutes of the universe, when there were still free neutrons available. Tritium and helium-three had to wait until after the first one or two minutes when there were enough deuterium nuclei to make collisions frequent between deuterium and either a proton or a neutron. Helium-three is stable but turns into helium-four when it collides with a neutron. Lithium-six usually comes from a collision between helium-four and deuterium. A collision between helium-three and tritium would also make lithium-six but the two parts would have to come together before the weak force turned the tritium into helium-three. If a helium-four nucleus hit one of the available tritium nuclei first that would make lithium-seven.
In this way protons and neutrons formed the nuclei of the three elements of lowest weight: hydrogen, helium, and lithium. The formation of lightweight nuclei was almost complete in the first four minutes after creation.
The above scenario agrees in detail with the relative abundances of the low-weight elements. Many physicists have sought alternative scenarios, but none so far agrees so well. Therefore, we think that the above scenario is close to what really happened.
The First Halt in Nuclei Production
There are no stable combinations having a total of either five or eight nuclear particles. The lack of them impeded the formation of larger combinations. To make nuclei with nine or more nucleons the heat and pressure must last long enough for many collisions between the unstable combinations and the lighter elements. The rapidly falling temperature and pressure of the first morning prevented the formation of elements more complex than hydrogen, helium, and lithium while the early universe was cooling in the first few minutes.
Most collisions were between two nuclei. Triple collisions can make heavier elements. For example, ordinary helium (two protons and two neutrons) can make carbon (six protons and six neutrons) if three helium nuclei collide nearly simultaneously. But by the time there was enough helium to make triple collisions probable the mixture had cooled, and the collisions were not energetic enough to overcome the mutual repulsion of the positive charges on the nuclei.
The heavy nuclei could not be formed in the first morning. They had to wait until the morning of the second day, when low-mass elements kept on colliding for millions of years in the interiors of the first stars.
Insufficient Complexity
With only three elements, the mixture was chemically poor. Even after it cooled enough for the nuclei to capture electrons and form neutral atoms, very little chemistry was possible. Helium is a noble gas. That means that atoms of helium do not combine with anything. Hydrogen atoms combine with one another two at a time to form hydrogen molecules. Lithium combines with hydrogen to form lithium hydride, a colorless crystal. At the end of the first morning the only available chemical substances were atomic and molecular hydrogen, helium, metallic lithium, and lithium hydride. These were insufficient to form the complex combinations needed for life. No known form of life can exist with so few elements and so few chemical substances.
Conditions had to be very different on the first and second mornings to make all the elements. Conditions also had to be just right on the first morning. With too much temperature and pressure most of the neutrons might have combined with the protons to make deuterium. That would have equalized the number of protons and neutrons almost irreversibly. Deuterium is not suitable for making elements much heavier than sulfur (16 protons and usually 16 neutrons) because the heavier elements require more neutrons than protons for stability. When the single protons of ordinary hydrogen combine with other nuclei, they can turn into neutrons to supply the extra number of neutrons the heavy elements need. Deuterium doesn’t combine as easily as protons with other nuclei, and the proton of a deuterium nucleus doesn’t turn into a neutron easily. Without heavy elements there would be no iron (26 protons and usually 30 neutrons) to carry oxygen in the blood, for instance. On the other hand, if the pressure had been too low at the start or had not stayed high long enough, most of the neutrons would have decayed into protons. In that case there could never have been any elements heavier than hydrogen.
It was unlikely even under those early conditions for three particles to collide simultaneously. The first compound nucleus was deuterium, formed when a neutron hit a proton and the two nucleons stuck together. Almost all the deuterium that exists formed in the first four minutes of the universe, when there were still free neutrons available. Tritium and helium-three had to wait until after the first one or two minutes when there were enough deuterium nuclei to make collisions frequent between deuterium and either a proton or a neutron. Helium-three is stable but turns into helium-four when it collides with a neutron. Lithium-six usually comes from a collision between helium-four and deuterium. A collision between helium-three and tritium would also make lithium-six but the two parts would have to come together before the weak force turned the tritium into helium-three. If a helium-four nucleus hit one of the available tritium nuclei first that would make lithium-seven.
In this way protons and neutrons formed the nuclei of the three elements of lowest weight: hydrogen, helium, and lithium. The formation of lightweight nuclei was almost complete in the first four minutes after creation.
The above scenario agrees in detail with the relative abundances of the low-weight elements. Many physicists have sought alternative scenarios, but none so far agrees so well. Therefore, we think that the above scenario is close to what really happened.
The First Halt in Nuclei Production
There are no stable combinations having a total of either five or eight nuclear particles. The lack of them impeded the formation of larger combinations. To make nuclei with nine or more nucleons the heat and pressure must last long enough for many collisions between the unstable combinations and the lighter elements. The rapidly falling temperature and pressure of the first morning prevented the formation of elements more complex than hydrogen, helium, and lithium while the early universe was cooling in the first few minutes.
Most collisions were between two nuclei. Triple collisions can make heavier elements. For example, ordinary helium (two protons and two neutrons) can make carbon (six protons and six neutrons) if three helium nuclei collide nearly simultaneously. But by the time there was enough helium to make triple collisions probable the mixture had cooled, and the collisions were not energetic enough to overcome the mutual repulsion of the positive charges on the nuclei.
The heavy nuclei could not be formed in the first morning. They had to wait until the morning of the second day, when low-mass elements kept on colliding for millions of years in the interiors of the first stars.
Insufficient Complexity
With only three elements, the mixture was chemically poor. Even after it cooled enough for the nuclei to capture electrons and form neutral atoms, very little chemistry was possible. Helium is a noble gas. That means that atoms of helium do not combine with anything. Hydrogen atoms combine with one another two at a time to form hydrogen molecules. Lithium combines with hydrogen to form lithium hydride, a colorless crystal. At the end of the first morning the only available chemical substances were atomic and molecular hydrogen, helium, metallic lithium, and lithium hydride. These were insufficient to form the complex combinations needed for life. No known form of life can exist with so few elements and so few chemical substances.
Conditions had to be very different on the first and second mornings to make all the elements. Conditions also had to be just right on the first morning. With too much temperature and pressure most of the neutrons might have combined with the protons to make deuterium. That would have equalized the number of protons and neutrons almost irreversibly. Deuterium is not suitable for making elements much heavier than sulfur (16 protons and usually 16 neutrons) because the heavier elements require more neutrons than protons for stability. When the single protons of ordinary hydrogen combine with other nuclei, they can turn into neutrons to supply the extra number of neutrons the heavy elements need. Deuterium doesn’t combine as easily as protons with other nuclei, and the proton of a deuterium nucleus doesn’t turn into a neutron easily. Without heavy elements there would be no iron (26 protons and usually 30 neutrons) to carry oxygen in the blood, for instance. On the other hand, if the pressure had been too low at the start or had not stayed high long enough, most of the neutrons would have decayed into protons. In that case there could never have been any elements heavier than hydrogen.