Metals are a type of materials that have been used by man since ancient times. This group of substances is very numerous, but they all have common physical characteristics, which are commonly called metallic properties.
The hardness among them is common, but not decisive. More specific are others that the softest metal possesses. These properties are determined by the peculiarities of their structure at the molecular level.
Properties of metals
Iron and its alloys (steel, cast iron), copper, aluminum… The use of these materials marked breakthroughs in scientific and technological progress at different stages of the development of civilization. Each of these metals has characteristics that give it a unique practical value. Common features for them are high thermal and electrical conductivity, plasticity - the ability to maintain integrity during deformation, metallic luster.
The damask blade that cuts through iron armor and the softest metal, on which traces remain from the slightest impact, have a similar internal structure. It is based on a crystal lattice, at the nodes of which are atoms withpositive 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 metallic bond between positive ions located at the nodes of the crystal lattice is carried out due to the attractive forces that arise in the "electron gas". The hardness, density, melting point of the metal depend on the concentration of this "gas".
Evaluation criteria
The answer to the question of which metal is the softest will always be the subject of discussion, unless the evaluation criteria are agreed upon and the very concept of softness is defined. The opinion about this characteristic of the material will be different for specialists in different industries. A metallurgist may understand softness as increased malleability, a tendency to accept deformation from abrasive materials, etc.
For materials scientists, it is important 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 generally recognized indicators proving its "record" characteristics. There are several techniques that aim to measure the softness of different materials.
Measurement methods
Most of the certified methods for measuring hardness are based on the contact action on the material under test, measured with precision instruments, from a harder body called an indenter. Depending on the type of indenter and on the methods of measurement, there are several mainmethods:
- Brinell method. The diameter of the imprint left by a metal ball when pressed into the surface of the test substance is determined.
- Rockwell method. The depth of indentation into the surface of a ball or diamond cone is measured.
- Vickers method. The area of the imprint left by a 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 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 19th century by the German Friedrich Moos. It is based on the scratching method, where 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 index, a place in the scale and a digital index are determined for the tested substance. The softest reference mineral is talc. It has a Mohs hardness of 1, and the hardest one, diamond, is 10.
Evaluation of hardness on the Mohs scale is based on the principle "softer - harder". It is possible to determine exactly how many times, for example, aluminum, which has an index on the Mohs scale of 2.75, is softer than tungsten (6.0), only with measurement results based on other methods. But to determine the softest metal in the periodic table, this table is enough.
The softest are alkali metals
From the Mohs mineralogical scale, it can be seen that the softest are substances related to alkali metals. Even mercury, familiar to many from the liquid from a thermometer, has a hardness index of 1.5. Softer than it are 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 Mohs hardness scale of 0.2.
The physical and chemical properties of alkali metals are determined by their electronic configuration. It differs only slightly from the structure of inert gases. An electron located on an external energy level has mobility, which determines high chemical activity. The softest metals are characterized by a special volatility, they are difficult to mine and preserve unchanged. They have violent chemical interactions with air, water, oxygen.
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The name "cesium" comes from the Latin caesius - "sky blue": in the spectrum emitted by a very hot 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, with a Brinell hardness index of 0.15 MN/m2 (0.015 kgf/cm2). Melting point: +28.5°C, so under normal conditions, at room temperature, cesium is in a semi-liquid state.
It's rareexpensive and extremely reactive metal. In electronics, radio engineering and high-tech chemical industry, cesium and alloys based on it are increasingly used and the need for it is constantly growing. Its chemical activity, the ability to form compounds with the highest electrical conductivity are in demand. Cesium is an important component in the production of special optical devices, lamps with unique properties and other high-tech products. At the same time, softness is not its most sought-after quality.
