Animated Solution for Chemistry - d and f-Block Elements: What is the correct order of the following elements with respect to their density ?

The density of transition metals is a fascinating topic because it perfectly illustrates the interplay between atomic mass and atomic volume. Let's dive into the 3d transition series and see how these elements stack up against each other!

To understand the trend, we must first recall the fundamental definition of density:

This simple equation tells us that density is directly proportional to mass and inversely proportional to volume. Therefore, to predict the density of an element, we need to analyze how its atomic mass and atomic volume change.

As we move from left to right across the 3d transition series (from Scandium to Copper), two key things happen: 1. Atomic Mass Increases: The number of protons and neutrons in the nucleus steadily increases, making the atoms heavier. 2. Atomic Volume Decreases: The effective nuclear charge increases because electrons are being added to the inner 3d subshell, which shields the outer 4s electrons poorly. This pulls the electron cloud closer to the nucleus, generally decreasing the atomic radius and, consequently, the atomic volume.

With the mass going up and the volume going down, the density naturally increases. This is why we observe a steady rise in density from Chromium ($7.19{\text{ g/cm}}^3$) to Iron ($7.80{\text{ g/cm}}^3$), Cobalt ($8.70{\text{ g/cm}}^3$), and finally Copper ($8.90{\text{ g/cm}}^3$).

But wait, there is a catch! If the trend continues, Zinc should have the highest density, right? Wrong.

Zinc breaks the trend dramatically. Let's look at its electronic configuration:

Zinc has a completely filled 3d subshell. Because it has absolutely no unpaired d-electrons, its d-electrons do not participate in metallic bonding. This results in exceptionally weak metallic bonds compared to the other transition metals.

Because the atoms are not held together as tightly, the atomic volume of Zinc puffs up significantly. This sudden increase in volume causes its density to drop sharply to $7.13{\text{ g/cm}}^3$, making it even less dense than Chromium!

Putting it all together, we can easily arrange the given elements in increasing order of their density. Zinc, due to its weak metallic bonding, has the lowest density. It is followed by Chromium, Iron, Cobalt, and Copper.

Understanding the underlying physics of electronic configurations and metallic bonding makes memorizing these trends completely unnecessary!