be in only one excited state at a time, the collection of atoms contains all possible excited states. The light emitted as these atoms fall to lower energy states is responsible for the spectrum. (b) Atomic absorption. When white light is passed through unexcited atomic hydrogen and then through a slit and prism, the transmitted light is lacking in intensity at the same wavelengths as are emitted in (a) The recorded absorption spectrum is also a line spectrum and the photographic negative of the emission spectrum.
Table . The Spectral Lines for Atomic Hydrogen Series n n Spectral Region Lyman Balmer Paschen Brackett Pfund , .... , .... , ....
, .... , .... Ultraviolet Visible Infrared Infrared Infrared becomes more and more complex for heavier atom. There are, however, certain features which are common to all line spectra, i.e., (i) line spectrum of element is unique and (ii) there is regularity in the line spectrum of each element.
The questions which arise are: What are the reasons for these similarities? Is it something to do with the electronic structure of atoms? These are the questions need to be answered. We shall find later that the answers to these questions provide the key in understanding electronic structure of these elements.
. Bohr’s Model for Hydrogen Atom Neils Bohr ( ) was the first to explain quantitatively the general features of the structure of hydrogen atom and its spectrum. He used Planck’s concept of quantisation of energy. Though the theory is not the modern quantum mechanics, it can still be used to rationalize many points in the Fig.
. Transitions of the electron in the hydrogen atom (The diagram shows the Lyman, Balmer and Paschen series of transitions) atomic structure and spectra. Bohr’s model for hydrogen atom is based on the following postulates: i) The electron in the hydrogen atom can move around the nucleus in a circular path of fixed radius and energy. These paths are called orbits , stationary states or allowed energy states.
These orbits are arranged concentrically around the nucleus. ii) The energy of an electron in