Question

Answer the following question.

Describe the diagonal relationship between Li and Mg with the help of two illustrative properties.

Answer 1

1.  
   1. The relative placement of these elements with similar properties in the periodic table is across a diagonal and is called a diagonal relationship.
   2. Lithium is placed in group 1 and period 2 of the modern periodic table. It resembles magnesium, which is placed in group 2 and period 3.
      

      Group	1	2
      Period 2	Li	Be
      Period 3	Na	Mg

2. Li and Mg show similarities in many of their properties
   e.g.
   a. Reaction with oxygen:
   1. Group 1 elements, except for lithium, react with oxygen present in the air to form oxides (M₂O) as well as peroxides (M₂O₂) and superoxides (MO₂) on further reaction with an excess of oxygen
      2. This anomalous behaviour of lithium is due to its resemblance to magnesium as a result of the diagonal relationship.
      3. As group 2 elements form monoxides, i.e., oxides, lithium also forms monoxides.
      \[\ce{\underset{\text{Lithium metal}}{4Li}+ O2->[\Delta]\underset{\text{Lithium monoxide}}{2Li2O}}\]
      \[\ce{\underset{\text{Magnesium metal}}{2Mg} + O2->[\Delta]\underset{\text{Magnesium monoxide (oxide)}}{2MgO}}\]
      b. Reaction with nitrogen:
      1. All the group 1 elements react only with oxygen present in the air to form oxides, while group 2 elements react with both nitrogen and oxygen present in the air, forming corresponding oxides and nitrides.
      2. However, lithium reacts with oxygen as well as nitrogen present in the air due to its resemblance with magnesium.
      \[\ce{\underset{\text{Lithium}}{6Li} + N2->[\Delta]\underset{\text{Lithium nitride}}{2Li3N}}\] \[\ce{\underset{\text{Magnesium}}{3Mg} + N2->[\Delta]\underset{\text{Magnesium nitride}}{Mg3N2}}\]