Explain the diagonal relation ship between Li and Mg

On moving diagonally in the modern periodic table the elements show certain similarities. This is due to the fact that on moving across a period from left to right electropositivity of the atom decreases.On the other hand on moving down the group electropositivity increases. These two factors work in opposite direction. Hence the elements that are present diagonally to each other have similar properties.

Diagonal relationship is more noticeable in the second and third period of the modern periodic table.It is shown as under:-

As a result of diagonal relationship the physical properties such as the atomic size, electronegativity, density etc. and chemical properties of compounds such as reaction with water, nitrogen, decomposition reactions etc. are similar.
For example: The similarities between Li and Mg as a result of diagonal relationship are as follows:
  • Both are harder and lighter than the other elements in the respective groups.
  • Both react slowly with water; oxides of both are less soluble in water; hydroxides of both decompose on heating.
  • Both form nitrides (Li3N and Mg3N2) by directly reacting with nitrogen.
  • Both do not form superoxide.
  • On heating, carbonates of both give oxides and CO2; hydrogen carbonates of both do not exist as solid.
  • Chlorides of both (LiCl, MgCl2) are soluble in ethanol, deliquescent, and crystallize as hydrates (LiCl.2H2O, MgCl2.8H2O).

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