Question

Metallic solids are always opaque because:
(A) They reflect all the incident light
(B) They scatter all the incident light
(C) The incident light is readily absorbed by the free electrons in a metal
(D) The energy band traps the incident

Answer 1

Hint: ‘Opaque’ word is used for materials which can reflect and absorb all the incident light falling on it. The metals exist in a structure where the atoms are embedded in a sea of free electrons.

Complete answer:
-A metal is a substance that has a lustrous appearance, is a good conductor of electricity and heat. It conducts electricity due to the presence of free electrons in it. Also metals have the ability to lose electrons and form cations or undergo oxidation while reducing others. They react with oxygen in the atmosphere to form metal oxides like ZnO, $C{u_2}O$, $Ti{O_2}$, etc which are basic in nature.
Metals are also highly malleable and ductile (they deform under the influence of stress without breaking).

Examples of metals are: Fe (iron), steel (an alloy), Ni (nickel), etc.
-In metals the force of attraction between the nucleus and the outermost electrons is much weaker than the force of attraction on the same electrons by other atoms in the solid or liquid metal. These electrons involved thus become delocalised making the metallic structure a collection of atoms embedded in a sea of relatively mobile electrons. This type of free electron structure is known as metallic bonding.
The strength of this metal bond is strongest between the transition metal atoms because they have the largest number of delocalised electrons.
- When an incident light falls on the metals, it gets absorbed by the large number of electrons floating in the metallic structure. These electrons absorb this incident light and increase their own kinetic energy. Hence they become opaque.
This absorption of energy causes the electrons to get excited and their movement to higher energy levels or from the valence band to the conduction band. This leads to an overlapping of the valence band and the conduction band or the partially filled valence band.
So, the correct option is: (C) The incident light is readily absorbed by the free electrons in a metal.

So, the correct answer is “Option C”.

Note: Metals not just absorb light but they also reflect light incident on them. The excitation of free electrons to higher energy levels and the lattice vibrations cause the absorption of incident light which makes them opaque. While metals also reflect incident light back which causes their shiny or lustrous appearance.