Emission Spectrum and Atomic Spectra

1.0Atomic Spectrum

An atomic spectrum is the set of electromagnetic radiation frequencies (or wavelengths) emitted or absorbed by an atom. Each element has a unique atomic spectrum, just like a fingerprint, because no two elements have the same electronic structure.

When an atom absorbs energy (heat, light, or electricity), its electrons jump from a lower energy level to a higher one. When these electrons return to their original level, they release energy in the form of light. If this light is passed through a spectroscope, a spectrum is obtained.

2.0Types of Atomic Spectra

Atomic spectra are classified into two main types:

• Emission Spectrum: Emission spectra are produced when atoms in an excited state return to lower energy levels, emitting photons with specific energies (and thus, specific wavelengths). These spectra appear as a series of colored lines on a dark background and are unique for each element.
• Absorption Spectrum: In an absorption spectrum, atoms in the ground state absorb specific wavelengths of white light, thereby moving to higher energy levels. The result is a series of dark lines on a continuous bright spectrum, with the positions of the dark lines corresponding to the energies absorbed.

3.0What is Emission Spectrum?

An emission spectrum is the spectrum of light emitted by atoms when their electrons drop from higher to lower energy levels. The energy lost during this transition is emitted as photons, each with a specific wavelength.

The emission spectrum appears as bright colored lines on a dark background. Each line corresponds to a definite wavelength and hence a definite energy change within the atom.

4.0Classification of Emission Spectra

Emission spectra are further classified based on their origin and appearance:

Line Spectrum

• Line spectra consist of sharp, distinct lines corresponding to particular wavelengths.
• Produced by individual atoms of elements (especially gases at low pressure).
• Each element’s line spectrum is unique and acts as a “fingerprint”.

Read More: Line Spectrum of Hydrogen

Band Spectrum

• Band spectra are made up of closely spaced lines that merge into bands.
• Typical of molecules, not isolated atoms.
• Commonly observed in molecular gases.

Continuous Spectrum

• Continuous spectra show a seamless blend of colors or wavelengths, with no distinct lines.
• Produced by hot solids, liquids, or dense gases (e.g., sunlight, incandescent bulbs).

5.0Emission Spectrum of Hydrogen Atom

Hydrogen, the simplest atom, was key to the discovery and understanding of atomic spectra.

Hydrogen Spectral Series

Hydrogen atom spectra consist of several series of spectral lines, each corresponding to electron transitions to a specific lower energy level:

Lyman Series:

• Transitions to (n=1) (ground state)
• Lies in the ultraviolet region
• $(n_2=2,3,4,\dots )$

Balmer Series:

• Transitions to (n=2)
• Visible region
• $(n_2=3,4,5,\dots )$

Paschen Series:

• Transitions to (n=3)
• Infrared region
• $(n_2=4,5,6,\dots )$

Brackett Series: Transitions to (n=4) (infrared)

Pfund Series: Transitions to (n=5) (infrared)

Rydberg Formula

The Rydberg formula mathematically expresses the wavelengths of spectral lines in hydrogen:

$\frac{1}{\lambda }=RH\left(\frac{1}{n{1}^2}-\frac{1}{n_2^2}\right)$

where:

• ($\lambda$) = wavelength,
• $(R_H)=Rydbergconstant((1.097\times 10^7;m^{-1}))$,
• $(n_1)=lowerenergylevel$
• $(n2)=higherenergylevel((n2>n_1)).$

6.0Bohr’s Atomic Model and Explanation of Atomic Spectra

Niels Bohr explained atomic spectra by proposing that:

• Electrons revolve in fixed, quantized orbits (energy levels) around the nucleus.
• Energy is absorbed or emitted only when an electron jumps between these orbits.
• The energy difference (($\Delta E$)) between initial and final levels equals the energy (($E=h\nu$)) of the emitted or absorbed photon.

$\Delta E=E2-E1=h\nu$

This model successfully explained the hydrogen emission spectrum and provided the theoretical basis for quantized energy levels.

7.0Applications of Atomic Emission Spectra

Atomic emission spectra have vital applications:

• Elemental Analysis/Spectroscopy: Identifying and quantifying elements in unknown samples using their unique spectral lines. Used in flame tests and emission spectroscopy.
• Astronomy: Determining the composition of stars and distant galaxies by analyzing their light.
• Environmental and Forensic Science: Detecting trace elements or metals in samples.
• Lighting and Plasma Physics: Understanding the behavior of gases in discharge tubes, fluorescent lamps, and neon lights.