Allotropy

 ( Chemistry ) 

• From p. 14: "Om det ock passar väl för att uttrycka förhållandet emellan myrsyrad ethyloxid och ättiksyrad methyloxid, så är det icke passande för de olika tillstånd hos de enkla kropparne, hvari dessa blifva af skiljaktiga egenskaper, och torde för dem böra ersättas af en bättre vald benämning, t. ex. Allotropi (af αλλότροπος , som betyder: af olika beskaffenhet) eller allotropiskt tillstånd ." (If it i.e., is also well suited to express the relation between formic acid ethyl oxide i.e., and acetic acid methyloxide i.e.,, then it i.e., is not suitable for different conditions of simple substances, where these substances transform to have different properties, and therefore should be replaced, in their case, by a better chosen name; for example, Allotropy (from αλλότροπος, which means: of different nature) or allotropic condition.)
• Republished in German: From p. 13: "Wenn es sich auch noch gut eignet, um das Verhältniss zwischen ameisensaurem Äthyloxyd und essigsaurem Methyloxyd auszudrücken, so ist es nicht passend für ungleiche Zustände bei Körpern, in welchen diese verschiedene Eigenschaften annehmen, und dürfte für diese durch eine besser gewählte Benennung zu ersetzen sein, z. B. durch Allotropie (von αλλότροπος , welches bedeutet: von ungleicher Beschaffenheit), oder durch allotropischen Zustand ." (Even if it i.e., is still well suited to express the relation between ethyl formate and methyl acetate, then it is not appropriate for the distinct conditions in the case of substances where these substances assume different properties, and for these, the may be replaced with a better chosen designation, e.g., with Allotropy (from αλλότροπος, which means: of distinct character), or with allotropic condition.)
• Merriam-Webster online dictionary: Allotropy. The term is derived .. After the acceptance of Avogadro's hypothesis in 1860, it was understood that elements could exist as polyatomic molecules, and two allotropes of oxygen were recognized as O₂ and O₃. In the early 20th century, it was recognized that other cases such as carbon were due to differences in crystal structure.

By 1912, Wilhelm Ostwald noted that the allotropy of elements is just a special case of the phenomenon of polymorphism known for compounds, and proposed that the terms allotrope and allotropy be abandoned and replaced by polymorph and polymorphism. From p. 104: "Substances are known which exist not only in two, but even in three, four or five different solid forms; no limitation to the number is known to exist. Such substances are called polymorphous. The name allotropy is commonly employed in the same connexion, especially when the substance is an element. There is no real reason for making this distinction, and it is preferable to allow the second less common name to die out." Although many other chemists have repeated this advice, IUPAC and most chemistry texts still favour the usage of allotrope and allotropy for elements only.Jensen 2006, citing Addison, W. E. The Allotropy of the Elements (Elsevier 1964) that many have repeated this advice.

Examples of allotropes include:

Most stable structure under standard conditions.

Structures stable below room temperature.

Structures stable above room temperature.

Structures stable above atmospheric pressure.
     

• Cerium, samarium, dysprosium and ytterbium have three allotropes.
• Praseodymium, neodymium, gadolinium and terbium have two allotropes.
• Plutonium has six distinct solid allotropes under "normal" pressures. Their densities vary within a ratio of some 4:3, which vastly complicates all kinds of work with the metal (particularly casting, machining, and storage). A seventh plutonium allotrope exists at very high pressures. The transuranium metals Np, Am, and Cm are also allotropic.
• Promethium, americium, berkelium and californium have three allotropes each.