The spectra of alkali metals exhibit several distinct features due to their electronic structure. Alkali metals are found in Group 1 of the periodic table, and include elements like lithium (Li), sodium (Na), potassium (K), rubidium (Rb), and cesium (Cs). Here are some key features of their spectra:

1. Simple Line Spectra

Single Valence Electron: Alkali metals have a single valence electron in their outermost shell, which leads to relatively simple emission and absorption spectra. The spectral lines result from transitions of this single electron between different energy levels.

2. Prominent Series of Spectral Lines

Series of Lines: The spectra of alkali metals often show a series of sharp lines in the visible region of the spectrum, corresponding to transitions between different energy levels. For example, sodium has well-known D-lines at approximately 589 nm and 590 nm.

3. Line Splitting

Fine Structure: Spectral lines can be split into finer components due to the fine structure caused by spin-orbit coupling. For instance, the sodium D-lines are split into two closely spaced lines (the D₁ and D₂ lines) due to this interaction.
Hyperfine Structure: In addition to fine structure, hyperfine splitting can occur due to interactions between the nuclear spin and the electron’s magnetic field. This is particularly observable in more sensitive spectroscopic measurements.

4. Spectral Lines in the Visible Range

Bright Visible Lines: Many of the prominent spectral lines of alkali metals fall in the visible region of the spectrum, making them easy to observe. For example, sodium's D-lines and potassium's lines are visible to the naked eye.

5. Characteristic Wavelengths

Specific Wavelengths: Each alkali metal has characteristic wavelengths for its spectral lines. These can be used for identification and are a result of the specific energy differences between electronic levels in each element.

6. Strong Absorption Lines

Intense Absorption: Alkali metals exhibit strong absorption lines, which are associated with the same electronic transitions observed in their emission spectra. This strong absorption is useful in spectroscopy for detecting these metals in various samples.

7. Difficulties with Line Broadening

Broadening Effects: Spectral lines of alkali metals can exhibit broadening due to various factors such as pressure broadening (collisions with other particles) and Doppler broadening (motion of the atoms or molecules). This broadening needs to be accounted for in precise spectroscopic measurements.

Examples of Alkali Metal Spectra:

Sodium (Na): The famous sodium D-lines (589.0 nm and 589.6 nm) are widely used in street lighting and spectroscopy.
Potassium (K): Displays strong lines in the violet and red regions (e.g., the K₁ line at 766.5 nm and K₂ line at 769.9 nm).
Lithium (Li): Shows prominent lines in the red and blue regions (e.g., the Li α line at 670.8 nm).
Alkali elements consists of single valence electron similar to H atom. The effective nuclear charge z=1, which is identical with free H atom. In case of alkali element the principal quantum number n is modified to effective principal quantum number due to presence of the inner core electron.
The alkali spectra can be grouped into four chief series of alkali spectra as
1. Principal series: This series arises from the transition between various highest p levels to lowest s levels. The wavenumber for this series is given by

2. Sharp series: This series arises from the transition from highest s level to the lowest p level. The wave number for this series is

3. Diffuse series: This series arises from the transition from various highest D level to lowest p level. The wave number expression for this series is given by

4. Fundamental series: This series arises from the transition from various highest F level to lowest D level. The wave number expression for this series is given by

Here 𝜇_s, 𝜇_p, 𝜇_D, 𝜇_F are the characteristics constant of sharp principal, diffuse and fundamental series. 1,2 3,4 represents the various orbital states for S, P, D, F respectively.

Explanation

The electronic configuration of alkali metal is such that the core of an inert gas is surrounded by s - electron. For example

The configuration is thus identical to hydrogen as the core electron do not play any part in optical spectra.

Let us discuss the characteristics of alkali spectra taking example of Na atom. The optical 3s electron of Na when excited, electron jumps to higher energy states such as 3P, 4s, 3D, ...etc depending upon the amount of excitation energy. When excited electron returns ti the ground level it give rise to all 4 chief series as

Sharp series: n²S -> 3²p; n = 4,5,6 ....

Principal series: n²p -> 3²S ; n= 3,4,5,.....

Diffuse series: n²D -> 3²p; n= 3,4,5...

Fundamental series; n²F -> 3²D ; n = 4,5,6,...

In summary, the spectra of alkali metals are characterized by simple line structures, prominent and sharp lines, specific wavelengths, and observable fine and hyperfine structures. These features arise from their electronic configurations and interactions, and are fundamental in many applications of atomic spectroscopy.

This note is a part of the Physics Repository

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