Emission Spectra | Types, Production, Uses, Examples

Definition of Emission Spectra

The emission spectra of a chemical element or chemical compound is the spectrum of frequencies of electromagnetic radiation emitted due to an atom or molecule transitioning from a high energy state to a lower energy state.

emission spectra

Emission spectra are obtained when the radiations emitted from substances that have absorbed energy (either by passing an electric discharge through a gas at low pressure or by heating the substance to high temperature) are analysed with the help of a spectroscope.

Atoms, molecules or ions that have absorbed radiations are said to be excited. When the radiations emitted by different substances are analysed, the spectrum obtained consists of sharp, well-defined lines, each corresponding to a definite frequency (or wavelength). 

Examples of Emission Spectrum

Examples of Emission Spectrum
  1. Light is emitted when an electric spark has heated the gases or vapours of chemical substances. The colour of the light depends upon the substance under investigation.
  2. Sodium or salt of sodium gives off yellow light while potassium or salt of potassium produces a violet colour. 
  3. When the platinum wire is dipped into a sodium nitrate solution and then inserted into a flame, the sodium atoms emit an amber yellow colour. Similarly, when indium is inserted into a flame becomes blue. These definite characteristics allow elements to be identified by their atomic emission spectrum.

Types of Emission Spectra

There are three types of emission spectrum:

1. Line Spectrum

Line Spectrum

The emission spectrum is obtained by analysing the radiation emitted by passing an electric discharge through hydrogen gas at low pressure. Such a spectrum consisting of lines of definite frequencies is called line spectrum or discontinuous spectrum.

The line spectrum is also known as the atomic spectrum. The pattern of lines in the spectrum of an element is characteristic of that element and is different from those of all other elements. In other words, each element gives a unique spectrum irrespective of even the form in which it is present.

For example, we always get two important lines at 589 nm and 589.6 nm in the sodium spectrum, whatever its source may be. It is for this reason that the line spectra are also regarded as the fingerprints of atoms.

2. Continuous Spectrum

Continuous Spectrum

It consists of unbroken luminous bands of all the colours from violet to red. These spectra depend only on the temperature of the source and are independent of the source’s characteristics.

Incandescent solids, liquids, carbon arc, electric filament lamps etc., give a continuous spectrum.

3. Band Spectrum

Band Spectrum

It consists of a number of bright bands with a sharp edge at one end but fading out at the other end. Band spectra are obtained from molecules. It is the characteristics of the molecule.

Calcium and Barium salts in a bunsen flame and gases like carbon dioxide, ammonia and nitrogen in the molecular state in the discharge tube give band spectra.

When the bands are examined with a high resolving power spectrometer, each band is found to be made of a large number of fine lines, sharp edges but spaced out at the other end. Using band spectra, the molecular structure of the molecule can be studied.

Production of Emission Spectra

Production of Emission Spectra

When an atom or molecule absorbs energy, the electrons are excited to a higher level. When the electron falls back to the lower energy level, light is emitted, which is equivalent to the higher and the lower states energy difference.

Due to the availability of multiple states of energy, an electron can undergo numerous transitions, each giving rise to a unique wavelength that comprises the emission spectrum.

Emission Spectrum of Hydrogen Atom

Emission Spectrum of Hydrogen Atom

The spectrum of the hydrogen atom can be obtained by passing an electric discharge through the gas taken in the discharge tube under low pressure. The emitted light is analysed with the help of a spectroscope.

The spectrum consists of a large number of lines appearing in different regions of wavelengths. Some of the lines are present in the visible region, while others are in ultraviolet and infra-red regions. In 1885, J.J. Balmer developed a simple relationship among the different wavelengths of the series of visible lines in the hydrogen spectrum.

The relationship is :

1/λ = ν (cm-1) = 109677 [1/22 – 1/n2]

n is an integer equal to or greater than 3(i.e., n = 3, 4, 5 …). It is known as the Balmer formula. The Balmer formula gives only the spectral lines in the visible region. This series of lines that appear in the visible region was named the Balmer series. Soon afterwards, a series of spectral lines of the hydrogen atom in different regions were discovered.

These lines in different regions were grouped into five different series of lines, each being named after the name of its discoverer. These are Lyman series, Balmer series, Paschen series, Brackett series and Pfund series. Lyman series appears in the ultra-violet region. Balmer series appears in the visible region while the other three series lie in the infra-red region.

Characteristics of Emission Spectra

Characteristics of Emission Spectra

The characteristics emission spectrum for a given element is the set of all the possible x-ray transition lines. Lines that correspond to different transitions from initial states having a vacancy in the same layer constitute a spectral series, for example, K, L, M etc.

Uses of Emission Spectra

Uses of Emission Spectra

There are some uses of emission spectrum:

1. The emission spectrum can be used to determine the composition of material since it is different for each element of the periodic table. One example is astronomical spectroscopy: identifying the composition of stars by analysing the received light.

2. All material, when hot, will emit light. Everyday examples abound the stove element in the kitchen, the metal filament in a lightbulb, and even the sun. By the end of the 1800s scientists were observing this phenomenon in their laboratories but could not explain it.

3. Because each atom has a distinct light fingerprint via its line emission spectra, scientists can use this to identify the elements present in samples both here on earth and far away. In order to do this, it is necessary to have excited electrons that are ready to emit.

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