The diagonal relationship exists between Lithium and Magnesium.

Explanation:
The diagonal relationship is a term used to describe the similarities in properties between elements that are diagonally positioned to each other in the periodic table. These elements share certain characteristics due to their similar electron configurations.

Lithium and Magnesium are diagonally placed to each other in the periodic table, which means they have similar electronic configurations and exhibit comparable chemical and physical properties.

1. Atomic Structure:
- Lithium is a member of Group 1 (Alkali metals) and is located in the second period of the periodic table, while Magnesium is a member of Group 2 (Alkaline earth metals) and is located in the third period.
- Both Lithium and Magnesium have low atomic masses. Lithium has an atomic mass of 6.94 g/mol, while Magnesium has an atomic mass of 24.31 g/mol.

2. Similarities in Properties:
- Both Lithium and Magnesium are metals and have low densities.
- They have low melting and boiling points.
- Both elements have a strong tendency to lose electrons and form positive ions.
- Lithium and Magnesium are both highly reactive, but Magnesium is more reactive than Lithium due to its larger atomic size and lower ionization energy.
- They both readily form compounds, such as lithium oxide (Li2O) and magnesium oxide (MgO).
- Both elements have a strong affinity for oxygen, and their oxides are white, solid substances.
- They exhibit similar periodic trends, such as increasing atomic radius and decreasing ionization energy down the group.

3. Differences:
- Lithium is a soft, silvery-white metal, while Magnesium is a harder, silvery-white metal.
- Magnesium has a higher atomic mass and atomic number compared to Lithium.
- Magnesium has a higher melting and boiling point compared to Lithium.

Overall, the diagonal relationship between Lithium and Magnesium arises from their similar electron configurations and results in comparable chemical and physical properties. This relationship helps in understanding the trends and patterns in the periodic table and the behavior of elements in different groups and periods.