Definition of Atomic Spectra
Each element has a characteristic spectrum through which it can easily be recognized. When a beam of light from the sun is passed through a prism, it splits into a series of colour bands known as the rainbow of colours: violet, indigo, blue, green, yellow, orange and red (remembered as VIBGYOR).
A similar spectrum is produced when a rainbow forms in the sky. This means that sunlight is composed of a collection of electromagnetic waves having different wavelengths.
The prism bends the light of different wavelengths to different extents. The red colour with the longest wavelength has deviated the least while the violet with the shortest wavelength has deviated the most.
The splitting of light into series of colour bands is known as dispersion, and the series of colour bands is called a spectrum. There is continuity of colours in this spectrum, i.e., one colour merges into the other without any gap or discontinuity, and such a spectrum is known as a continuous spectrum.
Discovery of Atomic Spectra
The systematic attribution of spectra to chemical elements began in the 1860s with the work of German physicists Robert Bunsen and Gustav Kirchhoff, who found that Fraunhofer lines correspond to spectral emission lines observed in laboratory light sources.
Characteristics of Atomic Spectra
The characteristics of the atomic spectrum are observed as
1. The atomic spectrum should be a pure line spectrum.
2. The atomic spectrum should be the emission band spectrum.
3. The atomic spectrum should be an absorption line spectrum.
4. The atomic spectrum should be the absorption band spectrum.
Types of Atomic Spectra
Unlike the spectrum obtained by analyzing the sunlight, the spectra of atoms are not continuous. The spectra of atoms consist of sharp, well-defined lines or bands corresponding to definite frequencies. There are two types of atomic spectra :
- Emission spectra
- Absorption spectra.
1. 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 analyzed with the help of a spectroscope. Atoms, molecules or ions that have absorbed radiations are said to be excited.
Example: light is emitted when an electric spark heats the gases or vapour of chemical substances. The colour of the light depends upon the substance under investigation. For example, sodium or salt of sodium gives off yellow light while potassium or salt of potassium produces a violet colour. When the radiations emitted by different substances are analyzed, the spectrum obtained consists of sharp, well-defined lines, each corresponding to a definite frequency (or wavelength).
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. Absorption Spectra
When continuous electromagnetic radiation (say white light) is allowed to pass through a gas or a solution of some salt and the transmitted light is analyzed, we obtain a spectrum in which dark lines are observed in an otherwise continuous spectrum. These dark lines
indicate that the radiations of corresponding wavelengths have been absorbed by the substance from the white light. Such a spectrum containing a few dark lines due to light absorption is known as the absorption spectrum. The dark lines of definite wavelengths are also characteristic of the substance.
It may be noted that these dark lines appear exactly at the same place where the lines in the emission spectrum appear. For example, the emission spectrum of sodium consists of two important yellow lines at 589 and 589.6 nm. On the other hand, when white light is passed through the vapour of sodium, we get dark lines in the absorption spectrum at 589 and 589.6 nm.
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 analyzed 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 visible lines in the hydrogen spectrum. The relationship is :
1/λ = ν (cm-1) = 109677 [1/22 – 1/n2 ]
where, 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.