What are soft metals



As Alkali metals the chemical elements lithium, sodium, potassium, rubidium, cesium and francium from the 1st main group of the periodic table are called. They are silver, shiny, reactive metals that have a single electron in their valence shell. Although hydrogen is in the first main group in most representations of the periodic table and in some cases has similar chemical properties to the alkali metals, it cannot be counted among these, as it is neither solid nor has metallic properties under standard conditions.

Explanation of the name

The name of the alkali metals is derived from the Arabic word "al kalja" for "potash", the old name for potassium carbonate obtained from wood ashes. Sir Humphry Davy presented the element potassium for the first time in 1807 by means of fused-salt electrolysis from potassium hydroxide. The latter he obtained from potassium carbonate. Hence, in English and French, potassium is used potassium called.

properties

Alkali metals are shiny metallic, silvery-white (exception: cesium has a gold tone with the slightest contamination), soft light metals. They can be cut with a knife. Alkali metals have a low density. They react with many substances, for example with water, air or halogens, sometimes extremely violently with strong heat generation. The heavier alkali metals in particular can self-ignite when exposed to air. They are therefore stored under protective liquids such as paraffin or petroleum (lithium, sodium and potassium) or in the absence of air in ampoules (rubidium and cesium).

As elements of the first group of the periodic table, they only have a weakly bound s-electron, which they willingly give up. Their first ionization energies and their electronegativities are correspondingly small. In compounds they occur predominantly as monovalent cations, although compounds are known in which these metals are present in anionic form (e.g. sodium, complexed with so-called cryptands).

The radius of the element atoms as well as the cations increases sharply with increasing mass number. Many other properties of the alkali metals behave analogously:

  • Decrease in ionization energy,
  • Increase in electron affinity (becomes less negative),
  • Decrease in electronegativity,
  • Increase in basicity.

Alkali metals and their salts have a specific flame color:

  • Lithium and its salts color the flame red
  • Sodium and its salts turn the flame yellow
  • Potassium and its salts turn the flame purple
  • Rubidium and its salts turn the flame red
  • Cesium and its salts color the flame blue-violet.

Because of this flame color, alkali metal compounds are used for fireworks.

Alkali metals are used in atomic physics because their particularly simple electronic structure makes them particularly easy to cool with lasers.

General responsiveness

Alkali metals react with hydrogen to form salty hydrides:

The thermal stability of hydrides decreases from lithium hydride (LiH) to cesium hydride (CsH). Alkali hydrides are inter alia. used as a reducing or drying agent.

Alkali metals react with oxygen to form solid, white oxides (lithium oxide), peroxides (sodium peroxide) and hyperoxides (potassium hyperoxide, rubidium hyperoxide, cesium hyperoxide):

 

The reaction with water to form hydroxides takes place with the release of hydrogen:

The reactivity increases sharply from lithium to cesium; from the potassium onwards self-ignition occurs.

The alkali metals react with halogens to form the corresponding alkali metal salts:

The reactivity increases from lithium to cesium and decreases from fluorine to iodine. Sodium hardly reacts with iodine and very slowly with bromine, while the reaction of potassium with bromine and iodine is explosive.

Halogenated hydrocarbons can remove the halogen from alkali metals with the formation of carbon and explosions:

With liquid ammonia, alkali metals produce intensely blue-colored solutions. These solutions, which consist of positive alkali metal ions and solvated electrons, are a very strong reducing agent and are used, for example, for Birch reduction.

hydrogen

Hydrogen, the first element of main group 1, is a non-metal under normal conditions. It is therefore not counted among the alkali metals, but has some properties in common with them. Like the alkali metals, it always occurs in a monovalent manner and, under extremely high pressure, is converted into a metallic high-pressure modification, metallic hydrogen. On the other hand, some alkali metals also have properties such as hydrogen under certain conditions, e.g. B. lithium as a gas consists of 1% diatomic molecules.

See also

Categories: Alkali Metal | Substance group