What Is an Allotrope? One Element, Completely Different Substances

An allotrope is one of several forms a single element can take in the same physical state, differing in how the atoms bond and arrange. Diamond has each carbon bonded to four others in a rigid three-dimensional lattice, with every bond equally strong in every direction, which is why it is extremely hard and does not conduct electricity, since no electrons are free to move. Graphite has each carbon bonded to three others in flat hexagonal sheets, with strong bonds within a sheet and weak forces between sheets, so the layers slide over each other, which makes it soft and a good lubricant, and the leftover electron per atom moves freely within a sheet, which makes it conduct. Same element, same atoms, entirely different materials, and the case is the standard demonstration that structure determines properties at least as much as composition does.

Allotropy is common among elements that can bond in more than one way:

• •Carbon, with diamond, graphite, graphene, nanotubes and the cage-like fullerenes, which is the richest case known
• •Oxygen, with the ordinary two-atom molecule and the three-atom form called ozone, which behave completely differently and matter enormously in the atmosphere
• •Phosphorus, with white, red and black forms differing so much that white ignites in air while red is used on matchboxes
• •Sulphur, with several ring and chain arrangements that convert into each other with temperature
• •Tin, with a metallic form and a brittle grey form that appears in the cold, historically blamed for crumbling organ pipes and buttons
• •Iron, with several crystal arrangements at different temperatures, which is the basis of steelmaking and heat treatment

Which form exists under given conditions is a matter of thermodynamics and of how fast conversion happens. One form is generally more stable at a given temperature and pressure, and at ordinary conditions graphite is the stable form of carbon while diamond is not, which means diamonds are converting to graphite continuously. The conversion is unimaginably slow, because rearranging the lattice requires breaking every bond and passing through a high-energy intermediate state, so the rate is negligible and diamonds are stable in practice despite not being stable in principle. That distinction between thermodynamic stability and kinetic persistence runs through chemistry and explains a great many everyday materials. High pressure favours the denser arrangement, which is why diamond forms deep in the Earth and why synthetic production uses extreme pressure, and why the alternative synthesis route deposits carbon atom by atom under conditions where the diamond structure is the one that grows.

Two neighbouring ideas are regularly mixed up with this one. Isotopes are atoms of the same element with different numbers of neutrons, which is a difference inside the nucleus rather than in how atoms are arranged, and isotopes of an element have essentially the same chemistry while allotropes have very different chemistry. Polymorphs are the equivalent phenomenon for compounds rather than elements, where the same chemical formula crystallises in more than one arrangement, which matters enormously in pharmaceuticals because different crystal forms of a drug dissolve at different rates and a manufacturing process that produces the wrong one can render a medicine ineffective. Amorphous forms, lacking regular structure altogether, sit awkwardly in the classification, and glassy carbon and amorphous phosphorus are sometimes counted and sometimes not. The unifying point across all three is that identical ingredients arranged differently give materials that behave differently.

The practical consequences are large and not confined to carbon. Ozone and ordinary oxygen have opposite significance depending on altitude, with ozone high in the atmosphere absorbing ultraviolet radiation and ozone at ground level being a harmful pollutant, and both are just oxygen atoms bonded differently. The iron allotropes are the foundation of metallurgy, since heating and cooling steel moves it between crystal structures that dissolve different amounts of carbon, and controlling those transitions is what hardening, tempering and annealing do. The carbon forms discovered in recent decades, including the single-atom sheets and the tubes, have properties that have driven substantial materials research. Phosphorus allotropes differ so much in reactivity that the switch from white to red in match manufacture was a significant public health improvement. In each case the element is unchanged and the arrangement decides what the substance does.

Diamond bonds each carbon to four neighbours in a rigid lattice and graphite bonds each to three in sliding sheets, which accounts for every difference between them. Ozone and ordinary oxygen, white and red phosphorus, and the iron crystal forms behind steelmaking are the same phenomenon. Graphite is the stable form at ordinary conditions, so diamonds are converting, unimaginably slowly.