Animated Solution for Chemistry - d and f-Block Elements: The sum of the total number of bonds between chromium and oxygen atoms in chromate and dichromate ions is .........

When we dive into the chemistry of the d-block elements, two of the most vibrant and frequently tested ions are the yellow chromate ion (${\text{CrO}}_4^{2-}$) and the orange dichromate ion (${\text{Cr}}_2{\text{O}}_7^{2-}$). This question asks us to find the total number of bonds between chromium and oxygen atoms in both of these ions combined.

At first glance, this might seem like a simple counting exercise, but there is a hidden trap that catches many students off guard.

Linkages vs. Bond Order

Don't make a silly mistake here. When a question in this context asks for the "number of bonds," it is specifically referring to the number of individual $\text{Cr}-\text{O}$ linkages (or connections), not the sum of the bond orders.

If we were to count a double bond as "two bonds," we would end up with a completely different (and incorrect) answer. Our goal is simply to count how many oxygen atoms are directly connected to each chromium atom.

Let's start with the chromate ion, ${\text{CrO}}_4^{2-}$.

Imagine the central chromium atom sitting in the middle of a tetrahedron. It is surrounded by and bonded to four oxygen atoms. Regardless of whether these bonds are drawn as single or double bonds in a resonance structure, there are exactly four distinct $\text{Cr}-\text{O}$ linkages.

Number of bonds in chromate = $4$

Now, let's look at the dichromate ion, ${\text{Cr}}_2{\text{O}}_7^{2-}$.

You can visualize this structure as two tetrahedral chromate units that have come together to share a single oxygen atom at one corner. This shared oxygen acts as a bridge between the two chromium atoms ($\text{Cr}-\text{O}-\text{Cr}$).

Let's count the connections for each chromium atom: - The first chromium atom is bonded to three terminal oxygen atoms and one bridging oxygen atom. That is $4$ linkages. - The second chromium atom is also bonded to three terminal oxygen atoms and the same bridging oxygen atom. That is another $4$ linkages.

Total number of bonds in dichromate = $4+4=8$

Now that we have decoded the structures, the final step is a simple addition.

While you are studying these ions, always remember their pH-dependent interconversion. In an acidic medium, the yellow chromate ion dimerizes to form the orange dichromate ion. Conversely, in a basic medium, the dichromate ion breaks apart back into chromate.

This equilibrium is a high-yield concept for JEE and NEET, so keep it locked in your memory!