Question

Choose the correct order of density of the alkali metals :

• A. \(Li < K < Na < Rb < Cs\)
• B. \(Li < Na < K < Rb < Cs\)
• C. \(Cs < Rb < K < Na < Li\)
• D. \(Li < Na < K < Cs < Rb\)

Answer 1

The Correct Option is A

To determine the correct order of density for alkali metals, we need to understand how the density varies with atomic weight in this group. Alkali metals are found in Group 1 of the periodic table, and their densities typically increase with an increase in atomic number. However, there are exceptions due to atomic structure and packing.

The given options for the order of density of alkali metals are:

1. \(Li < K < Na < Rb < Cs\)
2. \(Li < Na < K < Rb < Cs\)
3. \(Cs < Rb < K < Na < Li\)
4. \(Li < Na < K < Cs < Rb\)

The correct order of density for alkali metals is \(Li < K < Na < Rb < Cs\). Let's analyze how we can determine this order:

1. Alkali metals typically have low densities due to their large atomic sizes and the presence of only one valence electron.
2. When comparing densities, we see that:
   • Lithium (Li) is the lightest and therefore has the lowest density.
   • Potassium (K) is less dense than sodium (Na), even though it has a higher atomic number, due to a peculiar atomic packing in its crystal structure.
   • Sodium (Na) comes next after potassium.
   • Rubidium (Rb) and Cesium (Cs) are heavier alkali metals, and their densities increase accordingly.

Based on this, the densities order from lowest to highest is indeed \(Li < K < Na < Rb < Cs\).

Hence, the correct option is: Option 1: \(Li < K < Na < Rb < Cs\).