Question: how are atoms made?

Susan Cartwright answered on 22 Jun 2015:

Atoms consist of a nucleus, composed of protons and neutrons, surrounded by a cloud of orbiting electrons.

To make an atom, you first make the nucleus (at high temperature). As the temperature cools, electrons will bind to the nucleus, making a neutral atom.

There are 8 different ways to make atoms.

1. In the early universe, initially there were free protons (hydrogen nuclei) and neutrons. As the universe expanded and cooled, the neutrons bound with the protons to make simple atoms. The light isotopes H-2 (deuterium; 1 proton + 1 neutron), He-3 (2 protons + 1 neutron), He-4 (2 protons + 2 neutrons) and a trace of Li-7 (3 protons + 4 neutrons) were made in this way, a few minutes after the Big Bang. Because there were more protons than neutrons, and because He-4 is by far the most stable of these nuclei, this period made mostly H-1 (hydrogen) and He-4, in the ratio of about 12:1 by number, with the other species being very small amounts.

2. In stars, lighter elements fuse together to make heavier ones. This makes the elements from carbon up to iron. It cannot make the elements between helium and carbon, because they are less tightly bound than helium, so when made they either combine further to carbon or break up back to helium. Nor can it make the elements beyond iron, because fusing iron with anything else does not generate energy – it costs energy to do it.

3. Iron and the elements around it (the “iron peak”, roughly the elements from vanadium to copper) are made in supernova explosions. These are the elements with the most stable nuclei (greatest binding energy). In the very high temperatures of a supernova explosion, only the very stablest nuclei survive.

4. Elements heavier than iron are made in two ways. Both involve adding neutrons (being neutral, neutrons can sneak into a nucleus without being repelled by the Coulomb force). In the first case, which takes place in stars that are fusing helium to carbon, neutrons are added to nuclei one at a time. If the produced nucleus is unstable, it will radioactively decay by beta decay, converting one neutron to a proton. This makes the element one place higher in the periodic table. All elements from iron to bismuth can be made this way. Because the neutrons are added slowly, it is known in astrophysics as the s-process.

5. Neutrons can also be added rapidly (the r-process). This makes very unstable nuclei with too many neutrons, which then decay by repeated conversion of neutrons to protons until they become stable. Of naturally occurring elements, thorium and uranium must be made in this way, because they are heavier than bismuth. Of the elements between iron and bismuth, some are made more by the s-process, and some more by the r-process – the overall contributions of the two processes are about the same. We are not completely sure where the r-process takes place – it might be in the outer regions of supernova explosions, further from the explosion than where the iron peak elements are made.

6. The elements between helium and carbon (lithium, beryllium and boron) are made by a process called spallation. Basically, high-energy cosmic rays knock bits off carbon nuclei to make slightly lighter nuclei. This is the only way to make these elements. The elements immediately below the iron peak, like scandium and titanium, are also made partially by spallation, this time knocking bits off iron nuclei.

7. Some elements are made by radioactive decay. For example, radium occurs naturally because it is in the decay chain of uranium and thorium. Although radium is made by the r-process, its lifetime is too short for any star-produced radium to be still present on the Earth today.

8. Finally, we can make artificial elements by bombarding nuclei with protons or neutrons. This is how the elements beyond uranium are made: they do not exist naturally on the Earth. The elements promethium and technetium, which have no stable isotopes despite being only middle-weight elements, also have to be made artificially.