Animated Solution for Chemistry - d and f-Block Elements: Which of the following compounds is metallic and ferromagnetic?

When we think of metal oxides, we often picture powdery, insulating materials like rust. However, the world of transition metal oxides is far more exotic. Because of the complex interactions between the $d$-orbitals of the metal and the $p$-orbitals of oxygen, these compounds can exhibit a wild spectrum of electrical and magnetic properties.

Some are perfect insulators, some are semiconductors, and a rare few conduct electricity just as well as pure metals! Similarly, while many are paramagnetic (weakly attracted to magnetic fields) or diamagnetic (weakly repelled), an exclusive club of these oxides exhibits ferromagnetism—the ability to form permanent magnets due to the strong, parallel alignment of their internal magnetic spins.

To find our metallic and ferromagnetic champion, let's systematically evaluate the given candidates:

1. Titanium Dioxide (${\text{TiO}}_2$): In this compound, Titanium is in a $+4$ oxidation state, giving it a $d^0$ electron configuration. With absolutely zero unpaired electrons, it is strictly diamagnetic and acts as an electrical insulator. It's widely used as a white pigment in paints, but it certainly won't stick to your fridge.

2. Vanadium Dioxide (${\text{VO}}_2$): This is a fascinating material. It is paramagnetic and is famous in materials science for undergoing a dramatic metal-insulator transition at around $68^{\circ }\text{C}$. Below this temperature, it's an insulator; above it, it becomes metallic. However, it lacks the ferromagnetic alignment we are looking for.

3. Manganese Dioxide (${\text{MnO}}_2$): Commonly found in dry cell batteries, ${\text{MnO}}_2$ has unpaired electrons and is paramagnetic. It is a semiconductor, not a true metal, and it does not exhibit ferromagnetism at room temperature.

This brings us to our final candidate: Chromium Dioxide (${\text{CrO}}_2$).

${\text{CrO}}_2$ is a synthetic magnetic oxide that breaks the mold. In this compound, Chromium is in a $+4$ oxidation state, leaving it with a $d^2$ configuration. The crystal structure allows the $d$-orbitals to overlap sufficiently to form a continuous conduction band, making it a metallic conductor.

More importantly, the exchange interactions between the unpaired electrons are incredibly strong, forcing all the magnetic spins to align perfectly parallel to one another. This makes ${\text{CrO}}_2$ strongly ferromagnetic at room temperature.

Because of its brilliant combination of metallic conductivity and strong ferromagnetism, ${\text{CrO}}_2$ was historically the premium material of choice for manufacturing high-fidelity magnetic recording tapes (audio and video cassettes).

Therefore, the only compound that satisfies both conditions of being metallic and ferromagnetic is ${\text{CrO}}_2$. This is a classic, high-yield factual concept in inorganic chemistry that frequently appears in competitive exams. Lock this fact into your memory!