Introduction to Alkali Metals
Alkali metals form the group 1 elements and consist of elements lithium, sodium, potassium, rubidium, caesium and francium. They belong to the s-block of elements as their outermost electron enters the s orbital giving them the electronic configuration of ns1. As the alkali metals have only 1 electron in their valence shell, they readily lose it during chemical reactions, making them count amongst the most reactive elements on earth. Thus, they are highly electropositive and form compounds which are ionic in nature. In this article, we will talk about the different compounds of alkali metals and their general characteristics.
Oxides and Hydroxides
- The alkali metals react vigorously with the oxygen present in the air to form oxides (O2−), peroxides (O22-) and superoxides (O2–).
- Lithium burns in air to form lithium oxide, whereas sodium forms a mixture of sodium oxide and sodium peroxide. Potassium forms a mixture of peroxide and superoxide whereas rubidium and caesium form superoxide only.
- The reactivity increases down the group. While lithium, potassium and sodium only burn in air, rubidium and caesium readily catch fire.
- The stability of the peroxide and superoxide increases as the size of the cation increases.
- The oxides so formed, react with water to form hydroxides.
Li2O + H2O —> 2LiOH
Na2O2 + 2H2O —> 2NaOH + H2O2
2KO2 + 2H2O —> 2KOH + H2O2 + O2(g)
- Pure oxides and peroxides are colourless. Superoxides are colored and paramagnetic.
- The alkali metals are among the most electropositive elements. The halogens (fluorine, chlorine, bromine, iodine and astatine) are the most electronegative elements. They react to form alkali metal halides which are salts.
- The alkali metal halides have high melting points and are colourless crystalline solids. They are soluble in water. And the boiling and melting points decrease from fluoride to iodide.
- The halides are prepared by reacting oxide or hydroxide with the aqueous acid (HX).
- The reactions are highly exothermic and have high negative enthalpies of formation.
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