When one element can exist in the form of several simple substances with different properties and characteristics, this is called allotropy, or polymorphism. Metals also have this ability. Some of them can change their structure and qualities under the influence of certain factors, which is sometimes very useful in industry and other fields. We will tell you about which metals have polymorphism and how this is manifested.
Different forms of one
Allotropy is a phenomenon in which one chemical element can form several varieties of simple substances. They can differ significantly from each other in their physical and chemical properties, but at the same time they will consist of atoms of the same kind. For example, graphite and diamond are completely different: they have a different color, hardness, structure, but both consist of carbon atoms.
Such variants are called allotropic modifications, and they appear due to the different order of arrangement of particles in crystal lattices or the different structure of molecules. So, a molecule of two oxygen atoms (O 2 ) forms a simple oxygen substance, but if there are three oxygen atoms in it (O 3 ), then the substance ozone will appear.
The concept of “allotropy” was introduced into scientific terminology by Jens Jakob Berzelius in 1841, after he discovered that tartaric and grape acids have the same composition, but differ in properties. Today, more than 400 modifications are known, among which there are both non-metals and metals. The term "allotropy" is applied to all simple substances without being attached to their state of aggregation. For solids, both simple and complex, the term polymorphism is used.
The phenomenon of metal polymorphism
Allotropy is a common phenomenon, but it is far from characteristic of all elements. The ability to form modifications depends on the internal structure of the atom itself, and is inherent in those elements that change their valency under the influence of external conditions.
In metals, polymorphism manifests itself mainly under the influence of temperatures. If a substance is heated to a certain value, the ions and atoms in its crystal lattice will begin to rebuild, changing their position relative to each other. As a result, the substance will acquire other properties and go into another modification.
Each new form is indicated by a lowercase letter of the Greek alphabet, which is appended to the element name through a hyphen. The lower the temperature at which the modification is formed, the earlier the letter is in the alphabet.
Metal polymorphism is widely used in industry. Often this property of substances is used to create strong alloys. The allotropy slopes are about 30 metals. Temperature allotropy is characteristic of tin, iron, uranium, beryllium, titanium, cobalt. Zinc, antimony, cesium, mercury, gallium, lithium and cadmium pass into other forms under the influence of pressure.
Iron
Fe, or iron, is one of the most abundant elements in the earth's crust. In the periodic table, he stands at number 26. As a simple substance, iron is a light ductile metal with strong magnetic properties. It is often used in various fields of life, but in most cases it is used in the form of alloys with carbon, chromium, nickel, manganese and other metals.
Iron polymorphism appears in four forms:
- Ferrite (α-Fe) - up to a temperature of 769 ° C. It has a body-centered cubic lattice and ferromagnetic properties.
- β-Fe - above 769 ° C. It behaves like a paramagnet and differs from ferrite only in some cubic lattice parameters. Not always allocated in a separate modification and is considered as α-Fe.
- Austenite (γ-Fe) - above 917 ° C. It has a face-centered cubic lattice, worse than ferrite conducts current and heat, behaves like a paramagnet.
- δ-Fe - above 1394 ° C. Volumetricated cubic lattice.
Tin
In the periodic table, tin, or Sn, stands at number 50. It is a light, shiny, silver-white metal that lends itself well to melting and forging. Metal polymorphism also manifests itself at different temperature conditions:
- Gray tin (α-Sn) is below 13.2 ° C. The diamond-type cubic lattice, the substance exists in the form of a gray powder, manifests itself as a narrow-gap semiconductor.
- White tin (β-Sn) - above 13.2 ° . The familiar form of tin is the only one of all modifications that has practical application and is used in industry. It has a tetragonal lattice; it produces a characteristic crunch when bent. When cooled, it turns into α-Sn, losing density and crumbling. This process has been called the "pewter plague."
- (γ-Sn) - above 161 ° . It has a rhombic lattice, characterized by increased fragility and density.