Fluorine is so reactive it is found in two of the most dangerous chemicals on the planet. The world’s strongest acid is fluoroantimonic acid, which is a mixture of two molecules that contain fluorine: antimony pentafluoride and hydrogen fluoride. The strongest acid we are likely to come across in our everyday lives is sulphuric acid, which itself can be pretty nasty and capable of burning skin. Fluoroantimonic acid is close to 10 quadrillion times stronger than sulphuric acid (that’s 1 followed by 16 zeros). It reacts violently with almost anything and is extraordinarily difficult to store, requiring containers made of compounds like Teflon that cannot be corroded.
Chlorine trifluoride is a second fluorine-based compound that is highly reactive. It is made from three atoms of fluorine and one of chlorine that are covalently bonded. Its electrons are arranged in a way that makes it highly unstable. You can use it to burn asbestos, a compound that was widely and effectively used to prevent fires in the middle of the twentieth century. It can even set alight ash made from burning wood. Although highly reactive, the compound is used in rocket fuel and in the computing industry in small doses. In everyday life, it is considered safe at levels of 0.1 parts per million, which is very dilute. There was therefore considerable risk when 900 kilograms of pure chlorine trifluoride were produced in the 1950s. While being moved across a warehouse, the chemical was accidentally spilled, burning through 30cm of concrete and 90cm of gravel before all the molecules had reacted and the fire burned out.
Highly reactive molecules are rare in nature because they quickly react with other molecules to form something more stable. Life does not use highly explosive compounds like chlorine trifluoride, 1-diazidocarbamoyl-5-azidotetrazole or fluoroantimonic acid. Although life requires energy, and highly reactive molecules can easily react, releasing it, life has found much more manageable ways of using chemistry.
If physics is the universe’s way of turning energy into atoms, then chemistry is the cosmos’s way of transforming elements into life. Life is picky with which elements it uses. Iron makes up just less than a third of our planet’s weight, but less than 0.01 per cent of your body’s. Oxygen is the second most abundant element on Earth, making up 30 per cent of its mass, but it is even more abundant in the human body, making up 65 per cent of your mass. Silicon, magnesium, sulphur and nickel are some of the most abundant elements found on Earth, yet life uses them sparsely, with each making up only a fraction of a per cent of you. We might be made from star dust, but life has been selective with which bits it uses.
Carbon is an element that is absolutely essential to life on Earth, and likely to life elsewhere in the universe if it exists. By weight, you are 18.5 per cent carbon. A carbon atom has six protons and six electrons, and anywhere between two and sixteen neutrons. The majority of carbon atoms have six neutrons and come in the isotope known as carbon-12, with only two other isotopes, carbon-13 and carbon-14, found in nature. Carbon-14 is weakly radioactive, decaying into nitrogen-14, a property that archaeologists use to age ancient artefacts. All other isotopes of carbon have only been made in laboratories and are unstable.
When carbon is in its ground state, it has two electrons in its inner shell, and four in its second shell. It can consequently make up to four covalent bonds with other atoms by sharing electrons. Being a small atom, the covalent bonds that carbon forms are quite strong, allowing it to form stable compounds. Each atom can combine in several ways to create compounds with very different properties. For example, at high pressure, each carbon atom can form single bonds with four other carbon atoms in a triangular pyramid structure, a tetrahedron, to produce diamonds. In contrast, at low pressure (and the right temperature) graphite forms. In this molecule, which is grey-black in colour, each carbon atom joins with three other carbon atoms via a complex electron-sharing arrangement. A sheet of linked carbon atoms is created, and these sheets are held together in a stack with electrostatic bonds, structured in a similar way to a ream of paper.
Carbon atoms not only join with other carbon atoms but can also form covalent bonds with atoms of other elements, including some metals. In the natural world, carbon often bonds with nitrogen, hydrogen and oxygen atoms, and because it can form multiple covalent bonds it frequently forms the large molecules that are so central to life.
There is no molecule more synonymous with life than deoxyribonucleic acid. A single molecule of DNA can contain billions of atoms. Each DNA molecule contains some of the genetic code to assemble organisms, be they bacteria, plants, fungi or animals. These molecules are stable, and DNA does not react with other molecules to change its atomic structure, which is a good thing, because if it did, the self-assembly manual to build you and me would be rapidly lost. Many other of life’s molecules are similarly quite stable. We store energy in the form of molecules of fat that can be kept until they are needed to free up energy. They are stable in our cells until we need energy, and they can then be broken down. Proteins, which are the workhorses of life, are also stable, being able to repeatedly facilitate other reactions. The chemistry of life is all about controlling reactions to either break down molecules to generate energy, or to use energy to build cells and structures like bones and brains.
Like all chemical reactions, those needed for life have an activation energy. Molecules that easily react with one another form reactions with very low activation energies, while other reactions, such as splitting water molecules into their hydrogen and oxygen components, require more energy. Once a chemical reaction is underway, it can either give out energy or take it in from the environment. An energy-generating reaction releases heat and light and is termed exothermic. In contrast, one that uses heat is endothermic. If you burn something, you are creating an exothermic reaction, while photosynthesis, the chemical reactions that plants use to live and grow, is an example of an endothermic reaction because it uses photons, the energy carrier particles, to power the production of sugars that the plant needs to grow.
Life uses both exothermic and endothermic reactions, and it uses a neat trick to reduce the activation energy of many of them. It does this using a class of chemicals called enzymes. Enzymes are nearly always proteins, molecules that consist of long strings of smaller molecules called amino acids that are folded into large, complex structures. Trillions of enzymes are active throughout your body at any one time. They are responsible for joining molecules together to build your body, and also to break molecules apart when burning food to provide the energy you need to go about your daily business. Enzymes are remarkable, and without them life could not exist.
Enzymes are large and complex, with complicated three-dimensional shapes. They often have valleys and channels and odd-shaped protuberances. A small number of amino acids within these structures form something called a binding site, which is specific to each enzyme and is where the chemical reaction it facilitates occurs. The binding site allows the enzyme to form electrostatic bonds with a specific chemical that is key to the reaction the enzyme catalyses.
A catalyst is any compound that can speed up a chemical reaction, and they do this by making it easier for the chemical reaction to start by lowering its activation energy. Enzymes are the catalysts that life most frequently uses, but catalysts are also important in inorganic chemistry. The molecule that an enzyme breaks apart or builds is called the substrate, and the enzyme alters the substrate’s shape. Some enzymes manage the chemical reaction alone, while others use other molecules called cofactors. The enzyme binds to the substrate, lowers the activation energy of the reaction, triggers the reaction, and then releases the end products that might be new building blocks for your body, fuel to run each of your cells, or molecules to be disposed of. The enzyme itself is not chemically altered by the reaction and survives to repeat the reaction it catalyses again and again and again.