Last updated on August 29th, 2020 at 09:51 am
When a ray of light is passed through a glass prism, it is dispersed into a band of seven colors. This is called a spectrum.
When a gas or vapor is heated to a high temperature or subjected to electric discharge, it emits light of all wavelengths. This light, if passed through a prism, a spectrum is formed having single lines or band lines at definite positions and the lines have definite frequencies also. The line spectra are also formed by atoms and are called atomic spectra. The band spectra are actually a collection of many lines formed by a molecule and is also called a molecular spectrum. Line spectra are characteristic properties of elements.
Hydrogen atoms form a small spectrum. When an electric discharge is passed through pure hydrogen gas at very low pressure, radiation of a pink light is seen. This radiation when analyzed through a prism of a spectroscope, some distinct lines of colored lights are seen. These are the line spectra of hydrogen atom. The lines formed due to emission of radiation of brighter and are called emission spectrum or light spectra. And the lines formed due to absorption are darker and are called dark spectrum or absorption spectrum.
Later several lines were discovered in the spectrum of hydrogen. These are given in the following table.
Lyman series are the lines formed when an excited electron comes back to the 1st energy level from different energy levels. Balmer series are the lines formed when an electron comes back to the second energy level. Similarly, Paschen series, Bracket series and Pfund series are formed due to returning of electrons to 3rd, 4th and 5th series respectively.
Question may arise, that hydrogen atom has only one electron. Then how does it form a series of lines in its atomic spectra ? The only electron of hydrogen atom remains in the first orbit in the ground state. When hydrogen gas in a container is excited by heat or electric spark, the electron in the first orbit of different hydrogen atoms are promoted to different energy levels. When the energy source is cut off, the electrons return to the first orbit with emission of radiation. This radiation forms the lines on the photographic plate of spectroscope.