Alkali metal

The alkali metals are a family of highly reactive, soft, silvery metallic elements located in Group 1 of the periodic table. This group comprises lithium (Li), sodium (Na), potassium (K), rubidium (Rb), caesium (Cs), and francium (Fr). Together with hydrogen, which is also placed in Group 1 though chemically distinct, they make up the s-block elements whose outermost electron occupies an s-orbital. This characteristic electron configuration gives rise to a suite of closely related physical and chemical properties and makes the group one of the best examples of periodic trends.

Characteristic Properties

All alkali metals have one valence electron (ns¹), which is readily lost to form cations with a +1 charge. This results in:

• High reactivity, increasing down the group
• Softness—all can be cut with a knife, revealing a shiny surface that tarnishes rapidly
• Low density and low melting points compared with most other metals
• Vigorous reactions with water, producing hydrogen gas and strongly alkaline hydroxides

Caesium is the most reactive metal readily studied under laboratory conditions, although francium is expected to be more reactive theoretically. However, francium’s extreme scarcity and short half-life prevent practical comparison.
Because of their strong reactivity with oxygen, moisture, and in lithium’s case nitrogen, alkali metals must be stored under oil and are never found in nature as free elements. Naturally occurring compounds are widespread, especially those of sodium and potassium.

Occurrence, Abundance, and Prospects for Heavier Members

In order of natural abundance in Earth’s crust, the alkali metals occur as:sodium > potassium > lithium > rubidium > caesium > francium.
Francium is extraordinarily rare, existing only as a trace radioisotope produced transiently in certain radioactive decay chains. Its isotopes decay too rapidly for significant accumulation.
Efforts to synthesise element 119 (ununennium)—the next potential alkali metal—have thus far been unsuccessful, though active research continues. Theoretical studies predict that relativistic effects may significantly influence its behaviour, possibly causing deviations from typical alkali-metal chemistry.

Applications

Alkali metals and their compounds are highly useful:

• Lithium: employed in psychiatric medication and in lithium-ion batteries
• Sodium chloride: essential as table salt and historically vital as a trading commodity
• Sodium and potassium ions: critical biological electrolytes
• Rubidium and caesium: used in atomic clocks, with caesium clocks providing the standard definition of the second
• Sodium-vapour lamps: efficient light sources widely used in street lighting

While rubidium and caesium have no essential biological roles, they can exert both beneficial and harmful physiological effects.

Historical Discovery

Knowledge of sodium and potassium compounds stretches back to antiquity. Substances such as salt and potash were widely used long before the chemical distinction between their components was understood.

• In 1702, Georg Ernst Stahl suggested that sodium and potassium salts were fundamentally different.
• In 1736, Henri-Louis Duhamel du Monceau confirmed their distinct nature.
• The elements themselves were first isolated by Humphry Davy in 1807 through the electrolysis of molten potassium hydroxide and sodium hydroxide, marking the first successful extraction of metals using electrolysis.

Lithium was identified in 1817 by Johan August Arfwedson while analysing the mineral petalite. Jöns Jakob Berzelius named the element after the Greek lithos (“stone”), reflecting its discovery in minerals.
Later in the nineteenth century, spectroscopic analysis led Robert Bunsen and Gustav Kirchhoff to discover caesium (1860) and rubidium (1861) based on their distinctive emission lines—sky blue for caesium and deep red for rubidium.
The family of alkali metals played a key role in early periodic-table development. Their strikingly similar properties helped chemists such as Döbereiner and Mendeleev recognise recurring patterns among the elements.
Francium, the final alkali metal to be discovered, was identified in 1939 by Marguerite Perey at the Curie Institute. Working with purified actinium-227, she detected radiation inconsistent with known decay products and isolated a new element whose properties aligned with Group 1 chemistry. Francium is highly unstable and exists only in minute quantities.