Question:

The alkali metals form salt-like hydrides by the direct synthesis at elevated temperature. The thermal stability of these hydrides decreases in which of the following order?

Updated On: Jan 30, 2025
  • NaH > LiH > KH > RbH > CsH
  • LiH > NaH > KH > RbH > CsH
  • CsH > RbH > KH > NaH > LiH
  • KH > NaH > LiH > CsH > RbH
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The Correct Option is B

Solution and Explanation

As the size of alkali metal ion increases, lattice enthalpy decreases and hence the stability of the corresponding metal hydride decreases.
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Concepts Used:

Group 1 Elements

Group one of alkali metals is s-block elements with just one electron in their s-orbital. They are are alkali metals. They are named so because of the alkaline nature of the hydroxides and oxides.

Alkali metals are characterized by one s-electron in the valence shell of their atoms.

Alkali metals have a corresponding [Noble gas] ns1 electronic configuration. They occupy the first column of the periodic table. Alkali elements are:

  • Lithium(Li)
  • Sodium(Na)
  • Potassium (K)
  • Rubidium (Ru)
  • Cesium (Cs)
  • Francium (Fr)

They have occupied successive periods from first to seven. Francium is a radioactive element with very low half-life.

Electronic Configuration:

  • Alkali metals have one electron in their valence shell.
  • The electronic configuration is given by ns1. For example, the electronic configuration of lithium is given by 1ns1 2ns1.
  • They tend to lose the outer shell electron to form cations with charge +1 (monovalent ions).

This makes them the most electropositive elements and due to the same reason, they are not found in the pure state.