Wonders of the Universe

MATTER BY NUMBERS

Throughout human history the discovery and use of specific chemical elements has been intricately linked with the rise of civilisation. It is believed that copper was first mined and crafted by humans 11,000 years ago, and the specific characteristics of this metal ushered in a new age of technology and the transition from stone tools and weapons to metal ones. Four thousand years later it happened again but with iron which, even today, when mixed with carbon to form the alloy steel is the exoskeleton of industrial civilisation.

These two elements played a role in our history because of their particular physical characteristics. Copper was almost certainly the first metal to be used by humans; as it is such an unreactive chemical that it is one of the few metals that occurs naturally in its pure state. It is also very soft and malleable and so relatively easy to work into tools and weapons. When combined with another metallic element – tin – copper forms the alloy bronze; when combined with zinc it forms brass. Iron is, perhaps surprisingly, the most abundant element on Earth, and the fourth-most abundant element in the rocks of Earth’s crust. Although more difficult to extract and work with than bronze, iron is an excellent material for weapons manufacture as it is harder and lasts longer than bronze.

These two metals have had a profound influence on human history and sit just a couple of spaces apart in the periodic table. Iron (Fe) is element number 26 and copper (Cu) is at 29. The first humans to use these metals would, of course, have had no idea of the reason for the physical similarities and differences between the two elements. So what is the fundamental difference between them? The answer is remarkably simple. As described earlier, the atoms of each element are composed of three building blocks: protons, neutrons and electrons. We do not need to consider the quarks inside the protons and neutrons, because at the temperatures we encounter on Earth they stay locked away. So when discussing Earthly chemistry, we can ignore them.

THE SIX LIGHTEST ELEMENTS IN NATURE: HYDROGEN TO CARBON In each element the number of protons (p) in its nucleus is the same as the number of orbiting electrons, but the number of neutrons (n), which have no electric charge, can vary.

We have already encountered the first four elements; one of these, hydrogen, has an atomic nucleus consisting of a single proton. The proton has a positive electric charge, which allows it to trap an electron in orbit around it to form a hydrogen atom. The electron carries a negative electric charge, equal and opposite to that of the proton. This means that hydrogen atoms are electrically neutral. The reason why the electron has exactly the equal and opposite charge of the proton is not known. This is even more surprising when you look at the quarks that build up the proton. The proton is made up of three quarks – two up quarks and one down quark. The up quark has an electric charge of +2/3, and the down quark has a charge of -1/3. The electron has a charge of -1. So it is only when they are combined to form a proton that everything balances out properly. The neutron consists of two down quarks and an up, which means that it has no electric charge at all. This cannot be a coincidence, and it is one of the great challenges for twenty-first-century physics to explain it.

Chemical elements differ because of varying numbers of protons in their atomic nucleus, but the number of neutrons makes no difference to their chemical properties. Chemistry is down to the way the electrons behave that orbit around the nucleus, and the number of electrons is equal to the number of protons. As we know, the hydrogen atom consists of one proton and one electron, but there is another form of hydrogen called deuterium. Deuterium has a neutron attached to the proton inside its nucleus, but this doesn’t change its chemical properties as there is still only one electron. Technically speaking, deuterium and hydrogen are two different isotopes of the same element. Helium atoms always have two protons and two electrons; it also has forms with one and two neutrons, known as helium-3 and helium-4 respectively. Next comes lithium, with three protons, three electrons and either three or four neutrons, sometimes more. Carbon has six protons and varying numbers of neutrons, and so on. The rule is that each successive element has one more proton in its nucleus, and at least one more neutron, although the number of neutrons varies. The neutrons help the nucleus to stick together; which is bound tightly by the strong nuclear force, and neutrons add to this, even though they have no electric charge. Electric charge is a bad thing for the nucleus; because the protons are positively charged, they repel each other and try to blow the nucleus apart. The neutrons don’t suffer from this problem, which is one of the reasons why heavier nuclei tend to have more neutrons than protons.

So the construction of chemical elements is simple. If you want to turn iron into copper, add three protons and a handful of neutrons to its nucleus. That’s all there is to it. This is easier said than done, of course, yet nature can do it because when the Universe was only a few minutes old the first four chemical elements existed. The building blocks were present, but the heavier elements were assembled later