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.
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.
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.