Molecules
There were only a few bits of chemistry I liked at school, and those were the bits that resulted in explosions. Such violent reactions were few and far between, mainly because my teachers were all good chemists who made sure to give us chemicals that would not blow up when mixed. I did have a chemistry set at home, but it was hard to generate much chaos with that either. One of the reasons, as I eventually learned, is that the chemicals that are most explosive need to be kept in unreactive liquids, and for this reason were not included in chemistry sets and were kept locked up at school. Metals like sodium are kept in oil because as soon as they encounter air, they start to react with the oxygen in it. However, even the reactions of the most volatile chemicals are tame compared with the death of stars described in the last chapter, or the consequences of mixing matter and antimatter. Most of this chapter will be about reactions due to the electromagnetic force, and how electrons behave, organize themselves around atomic nuclei and form various types of atomic bonds. But before we start to focus on electrons, there is one other type of interaction to briefly consider as it too is central to our existence.
Shortly after our universe’s birth, when quarks, electrons and neutrinos came into existence, so too did particles of antimatter. These are called anti-quarks and positrons, and the anti-quarks can combine to form antiprotons and even antineutrons. Some of the most impressive explosions in the universe would have occurred when matter and antimatter collided, as they destroy one another and generate vast amounts of energy. For example, if you collide a negatively charged electron with its antimatter equivalent, a positively charged positron, they emit two highly energetic photons. Antimatter is rare in our universe because shortly after an antimatter particle is created it collides with a matter particle and they annihilate one another, creating energy. One unsolved mystery of science is why matter won out over antimatter early in our universe. Theory suggests that matter and antimatter should have been produced in equal amounts, and they should have instantly completely annihilated each other, leading to a very premature end to our universe. The theory must be lacking in some way because there was ever so slightly more matter than antimatter, and that led to our universe existing as it does today. If antimatter and matter had been produced in equal amounts, we would not exist.
Antimatter was not only formed at the birth of the universe. It is produced, sometimes in the most unexpected of places. The bananas in your fruit bowl create a steady stream of positrons, with each banana producing one positron every hour and fifteen minutes or so. Bananas contain an element called potassium, including a rare isotope of the metal named potassium-40, which very, very slowly decays via the weak nuclear force to calcium-40. If you had a gram of potassium-40, just under half of it would decay to calcium-40 in a billion years. We cannot predict when any particular atom of potassium-40 will decay via the weak nuclear force to calcium-40, but we know that on average it takes 1,290,000,000 years. Because this is an average, some potassium-40 atoms will decay much more quickly, while others may last for several billion years before decaying. Given that one atom of potassium-40 decays in each banana approximately every seventy-five minutes, this suggests that there must be a lot of potassium-40 atoms in each banana. There are. Just one gram of potassium-40 contains 15 followed by 21 zeros atoms. An average banana contains only about 0.015 g of potassium-40, but that is still a very large number of atoms. When a potassium-40 atom decays it releases a beta particle – in this case a highly energetic positron – which is annihilated when it encounters an electron. Our eyesight is not good enough to see the annihilation, but it does occur, and it releases energy. If we were to scale things up by annihilating 1 gram of matter with 1 gram of antimatter, it would produce the energy equivalent of the atomic bomb dropped on Hiroshima. We are fortunate that antimatter is not produced too rapidly in our fruit bowls. If it was, making a banana smoothie could be lethal. Antimatter plays no further role in the story of why you and I exist, but be thankful it is no longer very common in our universe. If it was, you and I would not be here.
Chemical reactions do not involve the annihilation of matter and antimatter. Instead of being driven by the weak nuclear force, they are determined by electromagnetism. Despite this, some chemical reactions can still be astonishingly violent. 1-diazidocarbamoyl-5-azidotetrazole, or azidoazide azide as it is sometimes called, is one of the most volatile molecules known to humanity. It is close to impossible to store, as a small change in pressure, exposure to light or even a tiny shift in temperature can cause it to explode. It is so unstable that standard instruments used to measure how volatile a chemical is cannot be used to measure it, as attempts to use them make it explode. At the other end of the spectrum, there are molecules that are very difficult to engage in chemical reactions. Helium, for example, is very stable and tends not to get involved in reactions with other substances. It consists of two protons, two neutrons and two electrons. This does not mean that helium is not useful; it has many uses. It is lighter than air such that helium balloons float, and it is also amusing to take a breath of helium and experience your voice jump up in register. What is it that makes some chemicals more reactive than others?
In the previous chapter I introduced the four fundamental forces but focused most on the nuclear forces. I described how the life cycle of stars fused hydrogen nuclei into helium nuclei, and helium nuclei into those of heavier elements. These nuclei then joined with electrons to create atoms of the heavier elements. The next key part of our history is the advent of molecules caused by elements joining together to form a huge array of chemicals. They do this via the electromagnetic force, and it is time for this force to take centre stage. The force drives all chemical reactions, from simple interactions between small molecules to the way that some drugs stop some complex molecules from doing their jobs inside your cells.
Although I didn’t particularly enjoy chemistry at school, it did play an important role in my early life. While I was growing up, my parents were drug dealers. They ran a chain of chemist shops in Cambridgeshire, dispensing prescriptions and selling over-the-counter treatments for minor ailments, illnesses and injuries. I did briefly toy with the idea of going into the family business, but I’d never enjoyed helping out in the shops as a teenager. It wouldn’t have been the right job for me. Given my parents were pharmacists, they were probably a little horrified by my chemistry reports from school. Nonetheless, as with my extracurricular physics reading, I spent time at home reading around the subject, but not in quite the same way as I did with physics. I failed to develop a good grasp of electromagnetism, but I did learn how molecules could treat and prevent disease. My parents had a large book listing endless ailments and diseases and the drugs that could be used to treat them. It was fascinating reading, and remarkable to learn how many different ways there were to become sick, and ways in which drugs can relieve symptoms. The problem was that every time I got a temperature, spot or rash, I would self-diagnose, always fearing the worst. Being pharmacists, my parents were very responsible with drugs and, apart from the occasional aspirin, my requests for drugs to treat scurvy, Guinea worm, elephantiasis or the bubonic plague fell on deaf ears.