Problem 106 - A Complete Resource Book in Chemistry for JEE Main

Question

The metallic lusture exhibited by sodium is explained by (a) diffusion of sodium ions (b) oscillation of loose electrons (c) excitation of free electrons (d) existence of body centred cubic lattice.

Step-by-Step Solution

Answer

The metallic lustre is due to (b) oscillation of loose electrons.

Step 1: Understanding Metallic Lustre

Metallic lustre is a shiny appearance observed in metals. This phenomenon typically arises from the interaction of light with the loosely held electrons in the metal.

Step 2: Identify Key Factor in Metals

Metals are known to have free or loosely-held electrons that can move or oscillate. These electrons play a significant role in the properties of metals, including their lustre.

Step 3: Oscillation of Electrons

The free electrons in metals can oscillate in response to light. This oscillation helps reflect light efficiently, causing the shiny appearance characteristic of metallic lustre.

Step 4: Review Provided Options

Now, review the given options and identify which one relates to the property of electrons causing the metallic lustre: (a) diffusion of sodium ions, (b) oscillation of loose electrons, (c) excitation of free electrons, (d) existence of body centred cubic lattice. Options (a), (c), and (d) are less related to the description of metallic lustre.

Key Concepts

Free Electrons

In metals, the outermost electrons are not tightly bound to their respective atoms. These electrons are referred to as *free electrons* because they have the ability to move relatively freely throughout the metallic lattice. This is a key characteristic that sets metals apart from non-metals.

The presence of free electrons contributes significantly to the physical properties of metals like electrical conductivity and thermal conductivity.

• The freedom of these electrons allows them to carry electric charge across the metal, making metals excellent conductors of electricity.
• Similarly, they can transfer thermal energy quickly, contributing to the efficient heat conduction in metals.

It's also these free-moving electrons that play a crucial role in giving metals their distinctive shiny appearance, known as metallic lustre. By being able to move freely, they can interact with light in a unique manner, which we will explore further.

Electron Oscillation

When light strikes the surface of a metal, the free electrons are prompted to oscillate. This oscillation is a reaction to the electromagnetic field of the incoming light wave.

• The electrons absorb energy from the light and start to vibrate at the same frequency.
• As they move back and forth, they quickly re-emit the absorbed energy in a process caused by oscillation.

This re-emission of light by oscillating electrons contributes to the intense, shiny appearance of metals.

The key point is that the oscillating electrons do not cling to the absorbed light energy. Instead, they act as tiny mirrors, reflecting a significant portion of the light. This efficient reflection is what we perceive as the metallic lustre.

Light Interaction with Metals

The interaction of light with metallic surfaces is quite fascinating due to the nature of free and oscillating electrons. When light interacts with the metal surface, several interesting phenomena occur:

• The first is reflection, where the metal surface bounces back light due to electron oscillation, giving metals their characteristic shine.
• Additionally, this interaction can cause metals to appear differently based on light sources and angles, contributing to the various optical properties metals can exhibit.

This interaction takes place because the free electrons modify the light waves. Metals tend to be opaque because these electrons distort and absorb portions of the light that attempt to pass through.

Furthermore, the manner in which light is re-emitted by metals is usually in phase with the incoming light, helping maintain the frequency and speed of the original light wave. These dynamic interactions highlight why metals appear distinctively lustrous compared to other materials.