Question

Alkali metal salts impart colour to flames with order the frequency of light released:
Li, Na, K, Rb, Cs.
A. - Cs < Na < K < Rb < Li
B. - Li < Na < K < Rb < Cs
C. - Li < K < Na < Rb < Cs
D. - Li < Na < K < Cs < Rb

Answer 1

Hint: The alkali metals and their salts when applied to Bunsen flame, they give a characteristic colour. The energy of flame causes an excitation of the outermost electron of the metal atom, which gives out the absorbed energy as visible light on, reverting back to its initial position. Li gives Crimson red, Na imparts Golden yellow, K gives Pale violet, Rb and Cs produce Violet colour .These different colours are due to different ionisation energy of alkali metals.

Complete step by step answer:
If we provide energy to any atom or ion by very strong heating, electrons of them can be promoted from their normal ground state orbital to their higher energy orbitals. As they fall back down to their lower levels (either in one go or in several steps), they release energy, which can be in the form of heat or light. Each of these jumps from excited to ground state orbital involves a specific amount of energy which corresponds to a particular wavelength (or frequency). Due to all these jumps, a spectrum of lines will be produced, some of which will be in the visible part of the spectrum. The color we see is a combination of all these individual colors.
Relation of energy released and frequency is:$E = h\nu $
In such way $E\alpha \nu $, i.e. energy released is directly proportional to the frequency
Hence, Energy released: Li < Na < K < Rb < Cs
Frequency released: Li < Na < K < Rb < Cs
So, the correct answer is “Option C”.

Note: Be careful about the relation of energy and frequency. The sequence of frequencies released will be the same as that is for energy.