Animated Solution for Chemistry - s and p-Block Elements: Two elements $A$ and $B$ have similar chemical properties. They don't form solid hydrogencarbonates, but react with nitrogen to form nitrides. $A$ and $B$, respectively, are

Imagine you are a chemical detective, and the problem has just handed you two crucial pieces of evidence about two mystery elements, $A$ and $B$.

First, they refuse to form solid hydrogencarbonates (also known as bicarbonates). Second, they have a special affinity for nitrogen, reacting directly with $N_2$ gas to form solid nitrides.

To crack this case, we need to scan the periodic table and see which groups exhibit these specific behaviors.

Let's start by interrogating the Group 2 elements, the alkaline earth metals like Magnesium ($Mg$) and Calcium ($Ca$).

These metals are known to be quite reactive with nitrogen upon heating. The reaction proceeds as follows: $3M+N_2\stackrel{\Delta }{\to }{M}_3{N}_2$

Furthermore, the bicarbonates of Group 2 metals are notoriously unstable in the solid state. If you try to evaporate the water from an aqueous solution of $Mg(HC{O}_3{)}_2$, it decomposes into magnesium carbonate, water, and carbon dioxide. They only exist happily in an aqueous solution: $MC{O}_3(s)+C{O}_2(g)+H_{2}O(l)\to M(HC{O}_3{)}_2(aq)$

So, it is highly probable that one of our mystery elements belongs to Group 2.

Now, let's look at Group 1, the alkali metals. Generally, metals like Sodium ($Na$) and Potassium ($K$) do not react directly with nitrogen gas.

Additionally, they form very stable solid bicarbonates. Think of baking soda, which is solid $NaHC{O}_3$. Based on this, Group 1 metals should be ruled out.

But wait! There is a famous rebel in Group 1: Lithium ($Li$).

Because Lithium is exceptionally small, its cation $L{i}^{+}$ has a very high charge density and polarizing power. This allows it to form a stable lattice with the highly charged nitride ion ($N^{3-}$): $6Li+N_2\stackrel{\Delta }{\to }2L{i}_{3}N$

Similarly, this high polarizing power distorts the large bicarbonate ion ($HC{O}_3^{-}$) so severely that solid $LiHC{O}_3$ cannot exist; it decomposes upon attempted isolation. Lithium perfectly matches our clues!

Why does Lithium, a Group 1 metal, behave so much like Magnesium, a Group 2 metal?

This is a beautiful manifestation of the diagonal relationship in the periodic table. As you move across a period from left to right, atomic size decreases and nuclear charge increases. As you move down a group, atomic size increases.

When you move diagonally (one step right and one step down), these two opposing effects roughly cancel each other out. This results in elements with very similar ionic potentials (charge-to-size ratios).

Because $L{i}^{+}$ and $M{g}^{2+}$ have similar polarizing power, they exhibit strikingly similar chemical properties. They both form nitrides, and neither forms a solid bicarbonate.

Therefore, our mystery elements $A$ and $B$ are Lithium ($Li$) and Magnesium ($Mg$).