Which one of the following pairs shows a diagonal relationship in the periodic table?

In the periodic table, a diagonal relationship refers to the similarity in chemical properties between elements positioned diagonally adjacent to each other in the second and third periods, typically from Group 1 to Group 2 and Group 13 to Group 14. One of the most well-known examples of this phenomenon is the relationship between lithium (Li) from Group 1 and magnesium (Mg) from Group 2.

Although lithium and magnesium belong to different groups, they exhibit strikingly similar chemical characteristics due to the balance of two opposing periodic trends — decreasing atomic size across a period and increasing size down a group. When these trends combine diagonally, they often result in comparable atomic radii, ionization energies, and electronegativities.

For instance, both lithium and magnesium form nitrides (Li₃N and Mg₃N₂) when heated with nitrogen, whereas other members of their groups do not readily do so. They also both form carbonates and phosphates that are sparingly soluble in water and decompose upon heating. Similarly, hydroxides and fluorides of lithium and magnesium are only slightly soluble in water, and their chlorides are anhydrous and soluble in organic solvents, unlike those of their heavier congeners.

The diagonal relationship occurs primarily because of their comparable charge densities (charge/ionic radius), which lead to similar polarizing power and bonding nature. This results in compounds of Li⁺ and Mg²⁺ ions showing partially covalent character rather than purely ionic.

Hence, among the given options, lithium and magnesium form the correct pair exhibiting a diagonal relationship, which is an important concept in understanding periodic trends and element classification within inorganic chemistry