Animated Solution for Chemistry - Periodicity in Properties: The set of elements that differ in mutual relationship from those of the other sets is

Navigating the Periods

When we first look at the periodic table, we are taught to read it in two simple ways: across the periods (horizontally) and down the groups (vertically). Elements in the same group share the same number of valence electrons, which dictates that they will have very similar chemical properties. For instance, all alkali metals in Group 1 are highly reactive and form $+1$ ions.

However, nature loves to throw a curveball. If we zoom in on the lighter elements of the $s$ and $p$ blocks—specifically the elements in the second and third periods—we notice a fascinating anomaly. The first element of a group often behaves less like its immediate vertical neighbor and more like the element situated diagonally below it to the right.

What is a Diagonal Relationship?

This cross-connection of properties is known as the diagonal relationship. It is most prominently observed in the first three groups of the $s$ and $p$ blocks:

1. Lithium (Li) in Group 1 is diagonally related to Magnesium (Mg) in Group 2. 2. Beryllium (Be) in Group 2 is diagonally related to Aluminum (Al) in Group 13. 3. Boron (B) in Group 13 is diagonally related to Silicon (Si) in Group 14.

These pairs share remarkable similarities. For example, both Lithium and Magnesium form normal oxides when burnt in oxygen, unlike the other alkali metals which form peroxides or superoxides. Similarly, both Beryllium and Aluminum have amphoteric oxides, meaning they can react with both acids and bases.

Why Does It Happen?

Why does this happen? It all boils down to a concept called polarizing power, which is mathematically expressed as the ratio of ionic charge to the square of the ionic radius:

Let's break this down. As you move from left to right across a period, the ionic charge increases and the ionic size decreases. This causes a sharp increase in polarizing power. Conversely, as you move down a group, the ionic size increases significantly while the charge remains the same, leading to a decrease in polarizing power.

When you move diagonally—one step to the right and one step down—these two opposing effects beautifully cancel each other out. The increase in charge is perfectly balanced by the increase in size. As a result, diagonally placed ions end up having almost identical polarizing power, charge-to-size ratios, and electronegativities, leading to strikingly similar chemical behavior.

Spotting the Odd One Out

Now, let's look back at our question. We are asked to find the set of elements that differ in their mutual relationship from the others.

- Option (a) is Li-Mg, which is a classic diagonal pair. - Option (b) is B-Si, another well-known diagonal pair. - Option (c) is Be-Al, the third major diagonal pair.

But what about Option (d), Li-Na? Lithium and Sodium are placed vertically in the exact same group (Group 1). They share a standard, expected group relationship, not an anomalous diagonal one. Therefore, the Li-Na pair is the odd one out, making it our correct answer.

Always remember to look out for these diagonal pairs when dealing with the chemistry of the second and third periods. They are a favorite trap in competitive exams!