Alkali metal
Alkali metals, the chemical elements lithium, sodium, potassium, rubidium, cesium, and francium in the first main group of the periodic table are referred to. They are silvery, reactive metals, which have a single electron in their valence shell. Although hydrogen is available in most versions of the periodic system in the first main group and in part similar chemical properties, such as the alkali metals, it can not be counted on this, since it is neither solid under standard conditions, nor has metallic properties.
Explanation of the name
The name of the alkali metals is derived from the Arabic word القلية / al - qalya for " potash ", the old name for potassium carbonate obtained from plant ashes, from. Sir Humphry Davy presented in 1807 for the first time the element potassium by a fused-salt electrolysis of potassium hydroxide dar. latter he won from potassium carbonate. In some languages , this is reflected in the name. So called potassium for example, in English and French and potassium potassio in Italian.
Properties
Alkali metals are shiny metallic, silvery- white (except for cesium has with minimal contamination golden brown ), soft light metals. They can be cut with a knife. Alkali metals have a low density. They react with many substances, such as water, air or halogens sometimes extremely violently under excessive heat. In particular, the heavier alkali metals may ignite spontaneously in air. Therefore, they are under protective liquids such as paraffin or petroleum ( lithium, sodium and potassium ) and stored under airtight conditions in ampules ( rubidium and cesium).
As members of the first group of the periodic table, they have only a weakly bound s- electron, they give off light. Your first ionization energies and their electronegativities are correspondingly small. In compounds, they are all almost exclusively as the monovalent cations, although even compounds are known, in which these metals are anionic ( eg Natride complexed with so-called cryptand ).
Alkali metals and their salts have a specific flame color:
- Lithium and its salts color the flame red
- Sodium and its salts color the flame yellow
- Potassium and its salts color the flame violet
- Rubidium and its salts color the flame red
- Cesium and its salts color the flame blue violet.
Because of this flame coloration alkali metal compounds for fireworks to be used.
In atomic physics, the alkali metals are used because they can be cool especially easy with lasers because of their particularly simple electronic structure.
All alkali metals crystallize in the body-centered cubic structure. Only lithium and sodium crystallize in the hexagonal closest packing when low temperatures prevail.
Trends
The radius of the element atoms and the cations increases with increasing mass number. Many other properties of the alkali metals exhibit a trend in the group from the top downwards:
- Decrease in the electronegativity
- Increase in reactivity,
- Decrease in the melting and boiling points,
- Decrease in the ionization energy,
- Increase in the basicity
- Decrease in hardness,
- Increase the density.
Responsiveness and compounds
The alkali metals react with hydrogen to form salt-like hydrides:
The thermal stability of the hydrides of lithium hydride takes ( LiH ) from the cesium hydride ( CSH). Alkali metal hydrides are among others used as reductant or desiccant.
The metals react with oxygen to form stiff, white alkali metal oxides ( lithium oxide ), alkali-metal ( sodium ) and alkaline metal oxides Hyper ( potassium superoxide, Rubidiumhyperoxid, Caesiumhyperoxid ):
The reaction with water to form alkali metal hydroxides takes place with liberation of hydrogen:
From lithium to cesium, the reactivity increases sharply; from the potassium carried out spontaneous combustion. The alkali metal hydroxides are colorless solids, which dissolve readily in water under strong heating, while reacting strongly basic. The hydroxides and their solutions are highly caustic.
The alkali metals react with halogens to form the salt-like alkali halides:
The reactivity increases from lithium to cesium and decreases from fluorine to iodine. As sodium reacts very slowly with iodine and hardly with bromine as the reaction of potassium with bromine and iodine occurs explosively.
Alkali metals can escape the halogen to form carbon and the corresponding alkali metal halide halocarbons under explosion phenomena:
Alkali metals with liquid ammonia yield an intense blue colored solutions. These solutions consists of positive alkali metal ions and solvated electrons is a very strong reducing agent and are used for example for the Birch reduction. A suitable complexing agent added to these solutions, corresponding salts with alkali metal anions, called Alkalide can form.
Hydrogen
Is hydrogen, the first element of main group 1, under normal conditions, a non-metal. It is therefore not counted among the alkali metals, but has some features in common with them. It occurs as the alkali metals always single-valued and transforms itself under extremely high pressure into a metallic high-pressure modification, the metallic hydrogen to. On the other hand, also have some alkali metals under certain conditions, properties such as hydrogen, for example, consists of lithium as a gas to 1% of diatomic molecules.