Allotrope

I

Introduction

Allotrope, two or more distinct physical forms of a chemical element in the same physical state. The term allotropy comes from the Greek allos tropos meaning “another shape.” Allotropes arise because of differing arrangements of an element’s atoms within its molecules or crystals.

One of the best-known examples of allotropy is carbon, which has multiple distinct allotropes including graphite, diamond, and buckminsterfullerene. Carbon atoms in diamond form a rigid, three-dimensional structure, with each carbon atom bonded to four other carbon atoms. In graphite the carbon atoms form stacks of flat honeycomb layers with only weak intermolecular forces between layers, while buckminsterfullerene forms balls and tubes with structures reminiscent of the geodesic domes designed by the architect Richard Buckminster Fuller.

Elements exhibiting allotropy include arsenic, antimony, iron, oxygen, phosphorus, selenium, sulfur, and tin.

II

Differing Properties of Allotropes

Because of their different internal structures, allotropic forms of an element may exhibit greatly differing values for such physical properties as color, luster, density, hardness, odor, and electrical and thermal conductivity. For example, diamond is extremely hard and does not conduct electricity, while graphite is much softer and does conduct electricity. Allotropes may also differ in chemical reactivity. Oxygen, another allotropic element, normally exists as an odorless gas necessary for life, each of whose molecules contains two atoms of oxygen. Sometimes, however, it exists as ozone, a highly reactive, poisonous gas with a sharp, pungent odor, each of whose molecules contains three atoms of oxygen.

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III

Types of Allotropy

There are two main kinds of allotropy, monotropy and enantiotropy. Monotropy (monotropic allotropy) occurs when one form of a substance is stable at all temperatures, while any other forms are metastable, and change (sometimes very slowly) into the stable form. For carbon, graphite is the stable monotrope, while diamond and buckminsterfullerene change, extremely slowly, into graphite. Phosphorus also exhibits monotropy: Red phosphorus is stable, while white (sometimes called yellow) phosphorus is metastable.

Enantiotropy (enantiotropic allotropy) occurs when one solid form of the substance changes into another solid form of the same substance when at a definite transition temperature. Tin, which changes from white tin to gray tin below 13°C (55°F), is an example of this type of allotropy. Gray tin is much more brittle than white tin. The Greek philosopher Aristotle recorded tin statues collapsing in the intense cold as long ago as the 4th century BC.

See also Isomer.

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