Metals - a type of material that has been used by man since ancient times. This group of substances is very numerous, but all of them have common physical characteristics, which are commonly called metallic properties.
Hardness among them is ordinary, but not determinative. More specific are others, which the softest metal also possesses. These properties are determined by the features of their structure at the molecular level.
Metal properties
Iron and its alloys (steel, cast iron), copper, aluminum ... The use of these materials marked the breakthroughs of scientific and technological progress at different stages of the development of civilization. Each of these metals has characteristics that give it unique practical value. Common signs for them are high thermal and electrical conductivity, ductility - the ability to maintain integrity during deformation, metallic luster.
A damask blade that cuts through iron armor, and the softest metal, on which traces of the slightest impact remain, have a similar internal structure. It is based on a crystal lattice, at the nodes of which are atoms with a positive and neutral charge, between which there is an "electron gas" - particles that have left the outer shells of atoms due to a weakening of the bond with the nucleus. A special metal bond between the positive ions located in the nodes of the crystal lattice is due to the attractive forces arising in the "electron gas". Hardness, density, melting point of a metal depend on the concentration of this "gas".
Criteria for evaluation
The answer to the question of which metal is the softest will always be the subject of discussion if the assessment criteria are not agreed upon and the very concept of softness is defined. Opinions about this characteristic of the material will be different among specialists in different industries. A metallurgist can understand softness as increased malleability, a tendency to perceive deformations from abrasive materials, etc.
It is important for material scientists to be able to objectively compare different characteristics of substances. Softness should also have generally accepted evaluation criteria. The softest metal in the world should have universally recognized indicators proving its "record" characteristics. There are several methods whose purpose is to measure the softness of various materials.
Measurement methods
Most certified methods of measuring hardness are based on the contact action of the material being measured with the help of precision tools from the side of a more solid body called an indenter. Depending on the type of indenter and on the measurement methods, several basic methods are distinguished:
- Brinell's method. The diameter of the imprint left by a metal ball is determined when pressed into the surface of the test substance.
- Rockwell Method. The depth of indentation into the surface of a ball or diamond cone is measured.
- Vickers method. The area of the print left by the diamond tetrahedral pyramid is determined.
- Shore hardness. There are scales for very hard and very soft materials - the depth of immersion of a special needle or the height of the rebound from the surface of a special striker is measured.
Mohs hardness scale
This scale for determining the relative hardness of minerals and metals was proposed at the beginning of the XIX century by the German Friedrich Moos. It is based on the scratch method, when a harder sample leaves a mark on a softer one, and is very convenient for finding out which metal is the softest. In relation to 10 reference minerals, which are assigned a conditional hardness indicator, a place in the scale and a digital index are determined for the test substance. The softest reference mineral is talc. It has a Mohs hardness of 1, and the hardest, a diamond of 10.
Mohs hardness rating is based on the principle "softer - harder". It is possible to determine exactly how many times, for example, aluminum, having an index on the Mohs scale of 2.75, is softer than tungsten (6.0), you can only have the results of measurements based on other methods. But to determine the softest metal in the periodic table, this table is quite enough.
The softest - alkali metals
From the Mohs mineralogical scale it can be seen that the softest substances are those related to alkali metals. Even mercury, familiar to many from the liquid from a thermometer, has a hardness index of 1.5. It is softer than several substances with similar physical, mechanical and chemical properties: lithium (0.6 on the Mohs scale), sodium (0.5), potassium (0.4), rubidium (0.3). The softest metal is cesium, which has a score of 0.2 on the Mohs hardness scale.
The physical and chemical properties of alkali metals are determined by their electronic configuration. It is only slightly different from the structure of inert gases. An electron located at an external energy level has mobility, which determines high chemical activity. The softest metals are characterized by special volatility, it is difficult to mine and keep them unchanged. They are characterized by violent chemical interactions with air, water, and oxygen.
Element No. 55
The name “cesium” comes from the Latin caesius - “sky blue”: in the spectrum emitted by a strongly heated substance, two bright blue stripes are visible in the infrared range. In its pure form, it reflects light well, looks like light gold and has a silvery yellow color. Cesium is the softest metal in the world, the Brinell hardness index is 0.15 Mn / m 2 (0.015 kgf / cm 2 ). Melting point: + 28.5 ° C; therefore, under normal conditions, at room temperature, cesium is in a semi-liquid state.
This is a rare, expensive and extremely chemically active metal. In electronics, radio engineering and the chemical industry of high technologies, cesium and alloys based on it are increasingly used and the need for it is constantly growing. Its chemical activity and ability to form compounds with the highest electrical conductivity are in demand. Cesium is an important component in the production of special optical devices, fixtures with unique properties and other high-tech products. At the same time, softness is not its most demanded quality.