Allotropy

Diamond and graphite samples with their respective structures. The lower right carbon formation is what is known as "grafeno", which is characterized by infinite carbon sheets of individual atoms.

Allotropy (change, rotation) is the property of some simple substances of having different atomic or molecular structures. Molecules made up of a single element and having different molecular structures are called allotropes.

• Oxygen. It can exist as atmospheric oxygen (OO)₂) and as ozone (O₃), which generates the distinctive penetrating smell in the vicinity of waterfalls.
• Phosphorus. It manifests as red phosphorus (Posphorus)₈) and as white phosphorus (Posphorus)₄), of different physical characteristics. Both have the same chemical formula, since what gives it different properties is its internal structure.
• Carbon. Alotropic varieties: graphite, diamond, graphene, nano carbon tubes, fullerene and coal.

Allotropy occurs due to the ability of some chemical elements to appear as various simple natural compounds, which are substances with different molecular structures and different or the same number of atoms. In general, changes in the state of aggregation of matter or its concomitant phenomena, such as temperature or pressure, are one of the most important factors that influence which allotropes of an element occur.

Variation in the properties of allotropes of an element are caused by differences in the molecular structures of these allotropes. For example, in diamond crystals, each carbon atom is linked to four neighboring atoms of this same element, which is why it adopts a tetrahedron-shaped arrangement that gives it a particular hardness. The hybridization of carbon orbitals in diamond is sp³.

In graphite, the carbon atoms are arranged in overlapping layers. In each shell they occupy the vertices of regular hexagons. In this way, each atom is bound to three of the same shell with more intensity and to one of the neighboring shell in a weaker way. In this case the carbon hybridization is sp². This explains the softness and oiliness –to the touch– of graphite. A pencil lead forms the line because, as it moves over the paper, a thin layer of graphite adheres to it.

Diamond and graphite, because they are two different simple, solid substances, made up of carbon atoms, are called allotropic varieties of the element carbon.

A third allotrope of carbon is fullerene (C⁶⁰) or buckminsterfullerene (in honor of the architect Buckminster Fuller, for having built the geodesic dome on the Île Sainte-Hélène, Montreal). Because it is shaped like a soccer ball, buckminsterfullerene is also known as a bucky ball.

List of allotropes

Nonmetal and metaloid

Metals

Among the metallic elements of natural origin (up to U, without Tc and Pm), 28 are allotropic under ambient pressure conditions: Li, Be, Na, Ca, Sr, Ti, Mn, Fe, Co, Y, Zr, Sn, La, Ce, Pr, Nd, (Pm), Sm, Gd, Tb, Dy, Yb, Hf, Tl, Po, Th, Pa, U. Considering only the relevant technology, six metals are allotropes:

Allotropic Structures

Among the common allotropic structures we have those of sulfur.

This nonmetal has a yellow, brown, or orange color. It is soft, brittle, light, gives off a characteristic rotten egg odor when combined with hydrogen, and burns with a blue flame, giving off sulfur dioxide. It is insoluble in water but dissolves in carbon disulfide. It is multivalent, and oxidation states -2, +2, +4, and +6 are common. In all states (solid, liquid and gas) it presents allotropic forms whose relationships are not completely known. Their common allotropic structures are:

• Rhombic sulfur: It is known in the same way as sulfur α. It is found of the stable transformation of the chemical below 95.5 °C (204 °F, the transition point), and most other forms are reversed to this modification if they are left to remain below this temperature. The rhombic sulfur has a lemon yellow color, water-soluble, slightly soluble in ethyl alcohol, dietary ether and benzene, and is very soluble in carbon disulfide. It has a density of 2.07 g/cm3(1.19 oz/in³), a hardness of 2.5 in the Mohs scale and the molecular formula it presents is S₈.

• Monoclinic sulfur: Also called prismatic sulfur or β sulfur. It comes as the stable modification of the element above the transition temperature while it is below the melting point.

• Cast sulfur: It is crystallized in prisms shaped like needles that are almost colourless. It has a density of 1.96 g/cm3(1.13 oz/in³), a melting point of 119.0 °C (246.7 °F) and the molecular formula that bears is S₈.

• Plastic sulfur: Also called gamma sulfur. It occurs when the melted sulfur at the normal boiling point or near it, is cooled to the solid state. This form ismorphic and is only partially soluble in carbon disulfide.

• Liquid sulfur: It has the notable property to increase its viscosity if the temperature rises. Its color changes to dark reddish black when its viscosity increases, and the darkening of color and viscosity achieves its maximum to 200 °C (392 °F). Above this temperature, the color is cleared and viscosity decreases.

At the normal boiling point of the chemical element (444.60 °C or 832.28 °F) gaseous Sulfur has an orange-yellow color. As the temperature rises, the color turns deep red and then lightens, at around 650 °C (202 °F), to a straw yellow.