Problem 75 - A Complete Resource Book in Chemistry for JEE Main

Question

Colour in transition metal compounds is attributed to (a) small size metal ions (b) absorption of light in uv region (c) complete (n, s) subshell (d) incomplete ( \(\mathrm{n}-1\) )d subshell

Step-by-Step Solution

Answer

Color in transition metal compounds is due to the incomplete (n-1)d subshell, causing d-d transitions.

Step 1: Identifying Transition Metals

Transition metals are elements found in the d-block of the periodic table, typically characterized by having an incomplete d subshell. They are known for forming colored compounds.

Step 2: Analyzing the Effect of Incomplete (n-1)d Subshell

The color in transition metal compounds is primarily due to the d-d electronic transitions. These transitions occur when electrons in an incomplete (n-1)d subshell absorb visible light, promoting an electron to a higher energy d-orbital.

Step 3: Evaluating Other Options

(a) Small size metal ions: This contributes to high charge density but not directly to color. (b) Absorption of light in the UV region: Transition metals typically absorb in the visible range, not just UV. (c) Complete (n,s)-subshell: This does not relate to the cause of color. It's the incomplete (n-1)d subshell that matters.

Key Concepts

Color in Compounds

The striking colors seen in transition metal compounds can capture our imagination. This occurs because of the unique way these compounds interact with light. These colors arise from the specific wavelengths of light they absorb. When light hits a transition metal compound, certain wavelengths are absorbed, while others are reflected or transmitted. This selective absorption and reflection give rise to the stunning colors we observe.
Understanding how and why these compounds exhibit color revolves around understanding the internal electron transitions within the metal ions. The phenomenon primarily involves the movement, or transition, of electrons between different energy levels within the metal's d subshell. The d-d transitions that result from this play a significant role, as they involve electron movement within the d orbitals, and thus are crucial to the compound's color identity.

d-d Transitions

Some of the most vibrant colors in chemistry come from d-d transitions. These transitions occur within the d orbitals of transition metal ions. Transition metals can exhibit d-d transitions because they typically have electrons in their d orbitals and incompletely filled d subshells. When these electrons absorb light, they may move (or "jump") to a different d orbital with higher energy.
This process of absorbing light and moving between orbitals is a "d-d transition." The specific wavelengths of light absorbed during these transitions are removed from the visible spectrum, leading to the characteristic colors we see. For instance, a compound might absorb green light, reflecting the other colors and causing the compound to appear red. In essence, d-d transitions define and dictate the specific hue a transition metal compound will exhibit.

Incomplete d Subshell

The incomplete d subshell is the defining characteristic when discussing the chemistry of transition metals. Transition metals possess at least one d orbital not completely filled with electrons. This unique feature allows for the d-d transitions that give these metals their diverse and vivid colors.
Each transition metal can have a distinct number of electrons in its d orbitals, differing from one element to another. This variance in electron configuration is the reason behind the wide array of colors in transition metal compounds. Since the d subshell isn't completely filled, there is room for electron movement which is crucial for the absorption of specific wavelengths of light and the resultant generation of color.

Periodic Table d-block

The d-block of the periodic table is a fascinating area where transition metals reside. This section includes metals known not just for their unique colors, but also for their remarkable chemical properties. Transition metals in the d-block are characterized by a gradual filling of the d orbitals, fundamentally influencing their chemical behavior.
The placement in the d-block means that these elements have properties bridging between those of the s-block and p-block, creating a versatility that defines the transition metals. This position in the periodic table gives rise to a mix of metallic and non-metallic properties, which incluences both the color of their compounds and their reactivity. This mix is why this area is so varied and captivating.