s-Block Elements

The s-block comprises the alkali metals of group 1 and the alkaline earth metals of group 2, highly electropositive elements whose chemistry is dominated by the loss of their outer s electrons.

Definition

The s-block elements are the metals of groups 1 and 2, in which the highest-energy electrons occupy an s subshell, characterized by low ionization energies, strongly ionic compounds, and fixed group oxidation states.

Scope

This topic covers the descriptive chemistry of groups 1 and 2: trends in atomic and ionic size, ionization energy, and reactivity; the characteristic +1 and +2 oxidation states; hydrides, oxides, peroxides, and superoxides; solubility and lattice-energy trends in their salts; the special chemistry of lithium and beryllium and the diagonal relationships; and solutions of the metals in liquid ammonia. It does not cover the s-block elements' biological roles in detail, treated under bioinorganic chemistry.

Core questions

• How do reactivity and size trend down groups 1 and 2?
• Why do these elements adopt fixed +1 and +2 oxidation states?
• What distinguishes lithium and beryllium from their heavier congeners?
• What are diagonal relationships and why do they arise?

Key concepts

• Alkali and alkaline earth metals
• Ionization energy and electropositivity
• Oxides, peroxides, and superoxides
• Lattice and hydration energy trends
• Diagonal relationships
• Solvated electrons in ammonia

Key theories

Periodic trends in the s-block

Down groups 1 and 2 atomic radius increases and ionization energy falls, making the heavier metals more reactive and their compounds more ionic, while across the two groups the higher charge of group 2 ions raises lattice and hydration energies.

Anomalous first-member behaviour and diagonal relationships

The small, polarizing lithium and beryllium ions show more covalent character and resemble magnesium and aluminium respectively, the diagonal relationship arising from similar charge-to-size ratios.

Oxides, peroxides, and metals in ammonia

Heavier alkali metals form peroxides and superoxides as well as normal oxides, and alkali metals dissolve in liquid ammonia to give blue solutions of solvated electrons, illustrating their strong reducing power.

Clinical relevance

The s-block supplies sodium and potassium for nerve and fluid physiology, calcium and magnesium for structure and enzymes, lithium for mood-stabilizing medicines, and reactive metals used industrially as reducing agents and in alloys.

History

Many s-block elements were first isolated by Humphry Davy through electrolysis of their molten salts in the early nineteenth century, and the alkali and alkaline earth metals were later identified spectroscopically by Bunsen and Kirchhoff, whose flame-spectroscopy work revealed several new members of the block.

Key figures

• Humphry Davy
• Robert Bunsen
• Gustav Kirchhoff

Seminal works

• greenwood1997
• weller2018
• housecroft2018

Frequently asked questions

Why do alkali metals become more reactive down the group?

Down group 1 the outer electron lies farther from the nucleus and is more shielded, so it is lost more easily; the falling ionization energy makes the heavier metals more reactive toward water and oxygen.

Why does lithium behave more like magnesium than like sodium?

The very small lithium ion has a high charge-to-size ratio similar to that of the magnesium ion, so the two share trends in covalency, solubility, and thermal stability of their salts, an example of a diagonal relationship.