Alkali Metals: Special properties of Lithium. Diagonal relationship with Magnesium.

Lithium is the first element in the alkali metal group. It is the lightest solid metal, soft, silvery-white, has a low melting point and is reactive, though not as reactive or as soft as the other alkali metals.

It has a high specific heat (calorific capacity), the huge temperature interval in the liquid state, high termic conductivity, low viscosity and very low density.

Diagonal relationship with magnesium
Lithium resembles magnesium because of their similar size.
Both Li and Mg are harder and have a higher melting point than the other elements in their respective groups.
Hydroxides and nitrates of both Li and Mg decompose when heated to form an oxide. Hydroxides of Li and Mg are both weak alkali.
Bicarbonates of Li and Mg are more soluble than their carbonates, while the other elements in their groups have more soluble carbonates.
Li and Mg have many more similar properties.

http://www.lenntech.com/periodic/elements/li.htm#ixzz2TuxsUbHP
http://www.transtutors.com/chemistry-homework-help/s-and-p-block-elements/diagonal-relationship-li-and-mg.aspx