By the late 1860s, nearly a century after the discovery of hydrogen, sixty-six elements were known, and had their weights estimated. Mendeleev, a Russian chemist, arranged them into the first version of the periodic table in 1869. He revised his table in 1871, and in so doing revised the weights of some elements and predicted the existence of three elements today known as scandium, gallium and germanium. The discovery of gallium and scandium in the late 1870s, and of germanium in 1886, led to Mendeleev’s periodic table becoming widely accepted. Chemistry had become a predictive science, and Mendeleev’s legacy as one of the greatest chemists of all time was guaranteed.
At the same time, Mendeleev was developing his periodic table, the first theories of how elements combined to make compounds were proposed, and work began in trying to understand the way atoms joined with one another via chemical bonds. The electron was discovered by J. J. Thomson in 1897, with key breakthroughs in radioactivity made by the Polish chemist Marie Curie and the New Zealand physicist Ernest Rutherford. Quantum mechanics, of which more soon, was formulated in the first half of the twentieth century, and by its second half, chemistry had become a mature science. Humanity had a good understanding of how and why chemical reactions worked, and they could even predict many of them.
The development of chemistry is a fabulous example of the application of the scientific method, but it also reveals that progress can take generations. Early observations were that some substances could react, for example with iron and silver seen to oxidize when exposed to water and air, and others could be broken down into constituent parts. The hypothesis that many things could be broken down into elements became refined, and experimentalists were able to identify many of the different types of elements. Mendeleev realized that many elements behaved in similar manners, and he was able to discover a way of grouping them together. This grouping suggested some underlying principles of the ways elements interacted, and chemists and physicists developed hypotheses of how these might work. Some of these hypotheses, such as the existence of phlogiston, an element found in all compounds that could burn, or the plum-pudding structure of the atom, where electrons were embedded in a soft, spongy, cake-like nucleus similar to a Christmas pudding, were proved wrong. But other hypotheses were supported, and ever more elegant, and technologically challenging, experiments were designed and conducted.
As with physics, mathematical equations were developed to capture chemistry and the workings of atoms and molecules. The electromagnetic force became well understood, and this allowed it to eventually be linked to the strong and weak nuclear forces in the Standard Model. Physics is arguably the most advanced of the sciences, with chemistry following a close second. There are still open questions in both subjects, and new breakthroughs and applications will continue, but major breakthroughs will likely happen at a slower pace than in the sciences that focus on life. We will discover new uses for molecules, and new molecules to solve humanity’s problems, and we will continue to unearth new nuances in the way that electromagnetism works, and hopefully link it to gravity as part of a theory of everything. But that’s for the future. Let’s now return to what we currently know, describing why atoms interact to form molecules that are essential for life.
Protons and neutrons, regardless of which element they form, are all made up of up and down quarks, so the nuclei of atoms of every element are made up of the same fundamental particles. Protons always have a positive electric charge, while the electrons that make the outer parts of atoms always have a negative electric charge. The difference in charge between protons and electrons means they are attracted towards one another. Despite each element being made up of the same type of positive and negatively charged particles, each element is different. It is difficult to get atoms of some elements, such as argon, to combine with other atoms via the electromagnetic force. In contrast, atoms of other elements, such as oxygen, are highly reactive, and will interact with atoms of other elements to form all sorts of compounds.
Not only do atoms of different elements behave differently when they encounter other atoms, but the physical properties of the elements also differ. Hydrogen and oxygen are gases at room temperature, mercury is a liquid, while lithium and iron are solid. When hydrogen and oxygen react to create water, the two gases combine to form a substance that is liquid. Unless, that is, the temperature is below zero degrees Celsius, in which case solid ice is formed, or above 100 degrees Celsius, when steam forms. These temperatures change with pressure, which is why at higher altitudes, where the air pressure is lower, steam is formed at temperatures lower than 100 degrees Celsius.
You are a complex form of chemistry, and to understand why you came into existence it is necessary to understand some chemistry. Millions of chemical reactions occur within each of your cells every second, and these reactions require atoms of elements that are reactive. If all elements were inert, like helium and neon, life could never have got started. The next step in our journey from the Big Bang to you is from atoms to molecules. If you found any of the concepts in the previous chapter strange, such that the universe has a maximum speed limit, or that gravity can slow time, or that the universe would look like a bubbling foam if it could be observed at the Planck scale, things continue to be peculiar at the quantum scale at which atoms share or exchange electrons.
In the world we experience, things do not suddenly disappear from one place and reappear in another. My dog, Woofler, does not suddenly vanish from the bedroom before instantaneously appearing in the garden, however much he might like to do that. Similarly, objects can only be in one place at a time. I cannot simultaneously be in the office and down the pub, however hard I try. The world of the very small inhabited by fundamental particles and atoms is rather different.
All electromagnetic radiation, such as visible light, travels in waves. Yet such radiation is made up of particles called photons. The world of the very small is strange because photons, and indeed other particles such as electrons, individual atoms and even small molecules, are simultaneously both particles and waves. Being two things at once doesn’t make much sense given our experience of always only being one thing: a body. However, believe it or not, you, the trees in the countryside, the cars on the road, the planets, and indeed the entire universe, are also both solid objects and waves. The waves these big objects form are billions of times smaller than the objects themselves and they do not influence our everyday lives, so we can ignore them. Things get peculiar when an object’s wave is bigger than the object itself, and this is what happens to tiny objects like photons, electrons and atoms. Objects with waves that are larger than they are exhibit what scientists refer to as quantum behaviour. One way to demonstrate this, a phenomenon also called wave-particle duality, is with a neat experiment.
Scientists have conducted the two-slit experiment numerous times, and they always get the same result: atoms can appear to simultaneously be in two places at once. To do the experiment you first need a device to fire individual atoms towards a screen that can record where they collide with it. The device and the screen are in fixed positions. Next, take a sheet of material with two, close together, parallel vertical slits cut into it and place it between the gun and the screen. The slits are wide enough for the atoms to pass through them. You now fire individual atoms at the sheet and record where they appear on the screen.