Alkali Metals | Physical Characteristics#

In this article, we will discuss the physical properties of alkali metals like density, melting and boiling points and characteristic flame test.

Density of Alkali Metals#

• We know that Density = Mass / Volume.
• Alkali metals have low density because of their largest size (volume) in the respective group.
• Density increases down the group with one exception, potassium is less dense than sodium.
• Density in increasing order: Li < K < Na < Rb < Cs
• Density increases down the group because increase in molecular mass outweighs increase in volume due to addition of protons and neutrons down the group.
• When we move from Na to K, effect of increase in volume is more pronounced as compared to effect of increase in atomic mass. Due to this potassium is less dense than sodium.

Melting and Boiling Points of Alkali Metals#

• Alkali metals have weak metallic bonding due to presence of only a single valence electron. Due to this reason, alkali metals have low melting and boiling points.
• Boiling and melting points of alkali metals decrease down the group due to decrease in metallic bonding.
• Decreasing order of boiling and melting points: Li > Na > K > Rb > Cs

Characteristic flame test of Alkali Metals#

• Alkali metals and their salts impart characteristic colour to an oxidizing flame.
• Due to heat from the flame, the valence electron is excited to higher energy level. When the excited electron comes back to the ground state, there is emission of radiation in the visible reason.

• Alkali metals when irradiated with light, the light energy absorbed may be sufficient to make an atom lose energy. This property makes cesium and potassium useful as electrodes in photoelectric cells.