Atomic and molecular emission and absorption spectra have been known for over a century to be discrete (or quantized). (credit: Unknown Author, via Wikimedia Commons) Mysteries of Atomic SpectraĪs noted in Quantization of Energy, the energies of some small systems are quantized. His many contributions to the development of atomic physics and quantum mechanics, his personal influence on many students and colleagues, and his personal integrity, especially in the face of Nazi oppression, earned him a prominent place in history. Bohr’s theory explained the atomic spectrum of hydrogen and established new and broadly applicable principles in quantum mechanics.įigure 30.13 Niels Bohr, Danish physicist, used the planetary model of the atom to explain the atomic spectrum and size of the hydrogen atom. From their sizes to their spectra, much was known about atoms, but little had been explained in terms of the laws of physics. For decades, many questions had been asked about atomic characteristics. In 1913, after returning to Copenhagen, he began publishing his theory of the simplest atom, hydrogen, based on the planetary model of the atom. Bohr became convinced of its validity and spent part of 1912 at Rutherford’s laboratory. The great Danish physicist Niels Bohr (1885–1962) made immediate use of Rutherford’s planetary model of the atom. Describe the triumphs and limits of Bohr’s theory.Illustrate energy state using the energy-level diagram.Explain Bohr’s planetary model of the atom.Explain Bohr’s theory of the hydrogen atom.Describe the mysteries of atomic spectra.It failed to make correct predictions of large-sized atoms and provided sufficient information only for smaller atoms.By the end of this section, you will be able to: Bohr’s model could not explain shapes and geometry molecules.However, according to Bohr, electrons move in a well-defined circular orbit. According to this principle, it is impossible to determine simultaneously the exact position and momentum of a small moving particle like an electron with absolute certainty. It does not obey Heisenberg’s uncertainty principle.This is based on de Broglie’s suggestion that electrons of light have a dual character. But modern research reveals that an electron moves around the nucleus in three-dimensional space. According to Bohr, the circular orbits of electrons are planar.It failed to explain the Stark effect when the spectral lines get split into fine lines under the influence of an electric field.It could not explain the Zeeman effect when the spectral lines are split into closely spaced lines under the influence of magnetic field.It could not explain the atomic spectra of elements having more than one electron.The Limitations of Bohr’s atomic model are as follows: Read Also: Planck Quantum Theory and Black Body Radiation Explanation for the simultaneous appearance of many spectral lines in the hydrogen spectrum: - Although hydrogen atom has only one electron (which can be excited to one higher orbit at a time) yet hydrogen spectrum consists of several spectral lines in different series of hydrogen spectrum such as Lyman, Balmer, Paschen, Brackett and Pfund series.If E 1 and E 2 are the energies of lower and higher energy levels respectively, then frequency (v) of the radiation emitted is given by the following relation: When the electron jumps back to the lower energy levels, it loses energy in the form of a quantum. The excited state is unstable and, therefore the electron has a tendency to come back to the ground state. The electron in that state is said to be in an excited state. When the energy is supplied from some external source, the electron may absorb energy and jump to a higher energy level. When the electron is present in the lowest possible energy level, it is said to be in ground state. Bohr’s theory has explained the atomic spectra of hydrogen atoms: - According to Bohr’s model of atoms, electrons in an atom can have only certain definite energy levels.Bohr’s theory has helped in calculating the energy of electrons in hydrogen atom and hydrogen like compounds: - On the basis of various postulates of Bohr’s model, it is possible to calculate the energy of an electron in nth orbit of hydrogen like compound.Thus, it removes the limitation of the Rutherford model of atoms. Thus, gradual loss of energy by the electron is not possible. Also, it cannot jump from the first orbit to the lower orbit as there is no orbit less than one. Bohr’s atomic model explains the stability of an atom: - According to Bohr’s theory, the electron does not lose energy as long as it revolves in a particular orbit.
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