Animated Solution for Chemistry - s and p-Block Elements: Both lithium and magnesium display several similar properties due to the diagonal relationship; however, the one which is incorrect is

When we look at the periodic table, we often expect elements in the same group to behave similarly. However, nature loves to throw in a twist. Elements placed diagonally across the second and third periods often share a striking resemblance in their chemical properties. This phenomenon is known as the diagonal relationship.

Lithium (Group 1) and Magnesium (Group 2) are the classic poster children for this relationship. But why does this happen? It all boils down to their ionic potential, which is the ratio of their charge to their ionic radius. Lithium has a small charge ($+1$) but is very small in size. Magnesium has a larger charge ($+2$) but is also larger in size. As a result, their charge-to-size ratios end up being remarkably similar. This gives them comparable polarizing power, leading to similar covalent character in their compounds.

Let's dive into the options provided in the question to see how deep this relationship goes. First, we look at their bicarbonates. Both lithium and magnesium form bicarbonates, ${\text{LiHCO}}_3$ and ${\text{Mg(HCO}}_3{)}_2$, which are highly soluble in water.

Interestingly, because of their high polarizing power, neither lithium nor magnesium can stabilize the large bicarbonate ion in a solid crystal lattice. Therefore, neither of them exists in the solid state; they can only be found in aqueous solutions. This is a perfect example of their shared behavior.

Next, let's consider their reaction with nitrogen. Most alkali metals do not react directly with nitrogen gas. However, lithium is the rebel of Group 1. It reacts directly with atmospheric nitrogen to form lithium nitride:

Magnesium, being an alkaline earth metal, naturally reacts with nitrogen to form magnesium nitride:

Once again, the diagonal relationship holds true. Both metals form nitrides directly.

What happens when we heat their nitrates? Most alkali metal nitrates decompose to give nitrites and oxygen. But lithium nitrate, owing to the high polarizing power of the ${\text{Li}}^{+}$ ion, strongly distorts the electron cloud of the nitrate ion. This causes it to decompose all the way to lithium oxide, releasing nitrogen dioxide and oxygen:

Magnesium nitrate behaves in the exact same way:

So, yielding ${\text{NO}}_2$ and ${\text{O}}_2$ on heating is another shared property.

Basic Carbonates

Finally, we arrive at the formation of basic carbonates. A basic carbonate is a complex salt containing both carbonate and hydroxide ions. Magnesium, when reacted with carbonate ions in water, forms a basic carbonate:

However, lithium does not form any basic carbonate. While it does form a normal carbonate (${\text{Li}}_2{\text{CO}}_3$), it lacks the specific chemical characteristics required to form a stable basic carbonate complex like magnesium does.

While the diagonal relationship beautifully explains the similarities between lithium and magnesium, it is not an absolute rule. The statement that "both form basic carbonates" is the incorrect one, making it the right answer to our question. Understanding these subtle exceptions is what truly masters inorganic chemistry!