THE HYDROGEN SOECTRUM

Atoms of hydrogen, formed on passing an electric discharge through hydrogen gas at a low emit, light which gives a spectrum of 4 bright lines: red, blue-green, blue and violet respectively in the visible region. These spectral lines were first studied by Ballmer and their wave lengths expressed by the Ballmer formula.

                             When the electron in the excited state comes back to the lower energy levels, it emits energy it had of absorbed earlier, in the form of photons of lights of specific frequency and hence of specific wavelength. Thus spectral lines of different series observed in the emission spectrum of hydrogen are due to the emission of energy of photons of different wavelength when the excited electrons in hydrogen atoms come back to lower energy levels. The wavelength of the spectral lines is dependent on the quantity of energy liberated. The greater the energy liberated, the shorter is the wavelength and hence greater is the frequency of the radiations emitted.

QUANTUM NUMBERS

A close examination of the spectral lines led to an extension of Bohr’s atomic model, and it was found that four quantum numbers are necessary to characterize the state of an electron in an atom. These are in fact the identification numbers for the electrons. First three quantum number provide the information related to the location of an electron of an atom resides. The four quantum numbers are:

1.       Principal quantum numbers(n):

                            It indicates the energy levels and the distance between the nucleus and the electron and it represent the size and energy of the electron orbit. It is denoted by n which can have non-zero positive integers.

2.       Azimuthal or subsidiary quantum numbers (l):

                            A line of atomic spectrum described by a value of ‘n’ is found to be composed of several closely packed discrete line in better resolving instrument.

3.       Magnetic quantum numbers(m):

                            A hyper line spectrum described by a value of l undergoes splitting to give fine lines in presence of strong magnetic field.

4.       Spin quantum numbers (s):

                            The orbital electron not only revolves around the nucleus, but also rotates about its own axis is known as spin.

BOHR’S ATOMIC MODEL:

Rutherford’s nuclear model simply stated that atom had a nucleus and the negative charge electrons were present outside the nucleus. It did not say anything as to how where these electrons were arranged. It also could not explain why electro did not fall into the nucleus. Niles Bohr  (1913) proposed his atomic model from the concept of nucleus as proposed by Rutherford but applied the Quantum Theory of Radiation, as developed by Planck and Einstein to explain the behavior of the planetary electrons, and was built on the basics of the following postulates:

1.       The electrons in an atom revolve round the nucleus in definite number of selected circular orbits. Each orbit is associated with a definite amount of energy and the energy of an electron remains constant so long as it stays in the orbit. Such selected orbits are called stationary stages or energy level or orbits.

2.       The absorption or emissions of energy take place always in whole number.

3.       Orbits are numbered as 1, 2, 3, etc are called K, L, M, etc respectively.

· DEFECTS OF BOHR ATOMI MODEL

Bohr atomic model had successfully explained the origin of hydrogen spectrum and was able to remove the defect of earlier Rutherford atomic model. However, it has the following drawbacks:

1.       No explanation for fine structure of spectra:-

                 When the hydrogen spectrum is observed with the help of spectroscope of high resolving power, it is found that the individual spectral lines are not really single but consists of several very fine lines lying close together. This fine structure of spectral lines in the hydrogen spectrum cannot be explained by Bohr atomic model.

2.        No explanation for multi electron system:-

                 Bohr’s atomic model only explained the origin of spectrum of hydrogen atom and hydrogen likes ion He, Li etc but it did not explain the origin of the spectrum of multi electron system.

3.        No explanation for dual nature of electron:-

                  Bohr’s atomic model has treated electron as particles of certain mass revolving around the nucleus in fixed orbits.

RUTHERFORD ATOMIC MODEL

Ernest Rutherford was born at New Zealand. He conducted research work with J.J.Thomson in 1895. Prof. Rutherford was awarded the Nobel Prize in 1908 for his researches in the field f atomic physics. Rutherford performed an experiment in order to explain the arrangement of sub-atomic particles in an atom. The experiment is known as alpha rays scattering experiment.

               It is based on alpha scattering particles. Having known that atom contains electrons and positive ion, Rutherford proceeded to perform this experiment to know as to how and where these were located in the atom. In 1911, Rutherford and Marsden performed their historic alpha particles scattering experiment using the apparatus. They directed stream of highly energetic alpha particles from radioactive source against a thin gold foil provided with a circular fluorescent zinc sulphide screen around it. Whenever alpha particles struck the screen, a tiny flash of light was produced at that point.