Question

In the gas phase, the colours of the halogens very remarkably:
Fluorine – Pale yellow
Chlorine – Pale greenish-yellow
Bromine – Deep red-brown
Iodine – Deep violet
As one moves from ${{\text{F}}_{\text{2}}}$ to ${{\text{I}}_{\text{2}}}$, the difference between the two energy levels:
(A) Decreases and probability of transition increases.
(B) Increases and probability of transition increases.
(C) Remains unchanged and probability of transition remains unchanged.
(D) Depends on the temperature.

Answer 1

Hint: We are given halogens. The halogens have seven valence electrons. Thus, they require only one electron to complete their octets. The halogens absorb radiations in the visible region and thus, the halogens are coloured.

Complete step by step solution:
We know that the halogens belong to group 17 of the periodic table. Halogens are non-metallic in nature.
The halogens have seven valence electrons. Thus, they require only one electron to complete their octets. Thus, halogens are more reactive than any other non-metals.
The halogens absorb radiations in the visible region. When halogens absorb radiation the valence electrons of halogens are excited to higher energy levels. Thus, the halogens are coloured.
The excitation of the valence electrons to higher energy levels differ for each halogen. Thus, each halogen has a different colour.
As we move from ${{\text{F}}_{\text{2}}}$ to ${{\text{I}}_{\text{2}}}$, the atomic radius increases. As a result, the energy levels come closer and the low energy light is absorbed and the colour becomes deep.
Thus, as one moves from ${{\text{F}}_{\text{2}}}$ to ${{\text{I}}_{\text{2}}}$, the difference between the two energy levels decreases and probability of transition increases.

Thus, the correct option is (A) decreases and probability of transition increases.

Note: Fluorine is smallest in size and thus, the effective nuclear charge is maximum. Thus fluorine absorbs high energy violet light and appears pale yellow in colour. Iodine has a large size and thus, the effective nuclear charge is minimum. Thus, iodine absorbs low energy violet light and appears deep violet in colour. Chlorine shows pale greenish-yellow colour and bromine shows deep red brown colour.