Physical properties of Alkali Metals

The table below summarises the physical properties of the Group 1 elements.

Element Melting Point
°C
Boiling Point
°C
Density
kg/m3
Atomic Radius
nanometer (nm)
Li 180 1342 535 0.15
Na 98 883 968 0.19
K 64 759 856 0.23
Rb 39 688 1532 0.25
Cs 29 671 1879 0.26

 

Alkali Metals have lower melting and boiling Points

All Group 1 elements have one electron in their outermost shell which is held very weakly by the nucleus. This electron can drift further from the nucleus than in most atoms of other elements. This results in Group 1 elements having larger atomic radii than those elements that follow them in their respective periods. The large atomic size results in weaker forces between neighbouring atoms. It is these weaker attractive forces due to the large atomic radii between neighbouring atoms of Group 1 elements that result in lower melting and boiling points when compared to other metals.

The decrease in melting and boiling points down the group can be explained by the additional shell being added to the previous element causing the atomic radius to increase. The increasing atomic radius means weaker forces between the atoms and so a lower melting and boiling point.

Alkali Metals are soft

Alkali metals are soft metals that can be cut with a knife and silvery coloured. Freshly cut alkali metals are shiny but tarnish rapidly due to reaction with oxygen in the air. They are generally stored under oil.

It is the reduced interatomic forces in these elements that make them relatively soft.

 

Alkali metals are extremely good conductors of electricity and heat

Alkali metals have one electron in their outermost shell which is held very weakly by the nucleus. The outer electron can drift further from the nucleus and move relatively freely. On the application of an electric field the large numbers of free flowing electrons in the outer shells conduct electricity through the metal.

Similarly, on the application of heat the free moving electrons in the outermost shells gain a lot of energy and transfer this through the metal via collisions with the other free moving electrons.

Alkali metals have low density

Lithium, sodium and potassium are less dense than water and therefore can float on water. Rubidium and caesium are denser than water.