Animated Solution for Chemistry - d and f-Block Elements: The set that contains atomic numbers of only transition elements, is

Imagine the periodic table not just as a chart, but as a sprawling map of a city. Every element has its own specific address, and the city is divided into four major districts or "blocks": the s-block, the p-block, the d-block, and the f-block.

When we talk about transition elements, we are specifically referring to the residents of the d-block. These elements are located right in the bustling center of the periodic table.

By definition, transition elements are those that possess partially filled $d$-orbitals in their ground state or in any of their common oxidation states. This unique electronic structure gives them their characteristic properties, like forming colored compounds and exhibiting variable oxidation states.

To quickly identify transition elements, it is incredibly helpful to know the atomic number boundaries of the $d$-block series. You don't need to memorize every single element, but knowing the "landmarks" will save you a lot of time.

The $3d$ series starts right after Calcium, running from Scandium ($Z=21$) to Zinc ($Z=30$).

The $4d$ series begins after Strontium, spanning from Yttrium ($Z=39$) to Cadmium ($Z=48$).

The $5d$ series is a bit trickier because the lanthanides interrupt it, but it essentially includes elements like Hafnium ($Z=72$) up to Mercury ($Z=80$).

Let's put on our detective hats and analyze the options provided in the question. We are looking for a set where every single atomic number belongs to the $d$-block.

Let's test the first option: $37,42,50,64$. Where does $Z=37$ land? It is Rubidium, an alkali metal sitting comfortably in the s-block. What about $Z=50$? That is Tin, a classic p-block element. And $Z=64$? That is Gadolinium, which is buried deep in the f-block as a lanthanide. Clearly, this set is a mixed bag. It is not our answer.

Now, let's examine the second option: $21,25,42,72$.

Watch closely as we map them out: - $Z=21$ is Scandium, the very first element of the $3d$ series. - $Z=25$ is Manganese, sitting proudly in the middle of the $3d$ series. - $Z=42$ is Molybdenum, a solid member of the $4d$ series. - $Z=72$ is Hafnium, which kicks off the rest of the $5d$ series right after the lanthanides.

Notice how every single one of these atomic numbers falls perfectly inside the boundaries of the $d$-block! There are no imposters here.

Since all the atomic numbers in this set belong exclusively to the $d$-block, they are all classified as transition elements.

This problem beautifully illustrates why understanding the general layout of the periodic table is far more powerful than rote memorization. By knowing the starting points of the blocks, you can easily deduce the nature of any element just from its atomic number.

Therefore, the correct set is indeed $21,25,42,72$.