CONCEPT OF BOHR'S ATOMIC THEORY By-N.K.D. Sir Please -Rate review and share this lesson
Planck's quantum theory When a black body is heated, it emits thermal radiations of different wavelengths or frequency. To explain these radiations, Max Planck put forward a theory known as Planck's quantum theory. The main points of quantum theory are: Substances radiate or absorb energy discontinuously in the form of small packets or bundles of energy. i) ii) The smallest packet of energy is called quantum. In case of light the quantum is known as photon. The energy of a quantum is directly proportional to the frequency of the radiation. E hv were v is the frequency of radiation and h is Planck's constant having the value 6.626x1027 erg-sec or 6.626x10 34 J-sec A body can radiate or absorb energy in whole number multiples of a quantum hv, 2hv, 3hv nhv where n is the positive integer. Nelis Bohr used this theory to explain the structure of atom. iii) v (or E- iv)
Bohr developed a model for hydrogen and hydrogen like atoms one-electron species (hydrogenic species). He applied quantum theory in considering the energy of an electron bond to the nucleus. Important postulates: An atom consists of a dense nucleus situated at the center with the electron revolving around it in circular orbits without emitting any energy. The force of attraction between the nucleus and an electron is equal to the centrifugal force of the moving electron. Of the finite number of circular orbits possible around the nucleus, and electron can revolve only in those orbits whose angular momentum (mvr) is an integral multiple of factor h/2T. nh 2T mvr where, m = mass of the electron v velocity of the electron n orbit number in which electron is present r radius of the orbit
As long as an electron is revolving in an orbit it neither loses nor gains energy. Hence these orbits are called stationary states. Each stationary state is associated with a definite amount of energy and it is also known as energy levels. The greater the distance of the energy level from the nucleus, the more is the energy associated with it. The different energy levels are numbered as 1, 2, 3, 4, (from nucleus onwards) or K, L, M,N etc. Ordinarily an electron continues to move in a particular stationary state without losing energy. Such a stable state of the atom is called as ground state or normal state. If energy is supplied to an electron, it may jump (excite) instantaneously from lower energy (say 1) to higher energy level (say 2, 3, 4, etc) by absorbing one quantum of energy. This new state of electron is called as excited state. The quantum of energy absorbed is equal to the difference in energies of the two concerned levels. Since the excited state is less stable, atom will lose it's energy and come back to the ground state Energy absorbed or released in an electron jump, (AE) is given by Where E, and E, are the energies of the electron in the first and second energy levels, and v is the frequency of radiation absorbed or emitted.
Note: If the energy supplied to hydrogen atom is less than 13.6 eV, it will accept or absorb only those quanta which can take it to a certain higher energy level i.e., all those photons having energy less than or more than a particular energy level will not be absorbed by hydrogen atom But if energy supplied to hydrogen atom is more than 13.6 eV then all photons are absorbed and excess energy appear as kinetic energy of emitted photo electron Merits of Bohr's theory i) The experimental value of radii and energies in hydrogen atom are in good agreement with that ii) Bohr's concept of stationary state of electron explains the emission and absorption spectra of ii The experimental values of the spectral lines of the hydrogen spectrum are in close agreement calculated on the basis of Bohr's theory. hydrogen like atoms with the calculated by Bohr's theory
Limitations of Bohr's Theory i) It does not explain the spectra of atoms or ions having more than one electron ii Bohr's atomic model failed to account for the effect of magnetic field (Zeeman effect) or electric field (Stark effect) on the spectra of atoms or ions. It was observed that when the source of a spectrum is placed in a strong magnetic or electric field, each spectral line further splits into a number of lines. This observation could not be explained on the basis of Bohr's model i de-Broglie suggested that electrons like light have dual character. It has particle and wave character. Bohr treated the electron only as particle iv) Another objection to Bohr's theory came from Heisenberg's Uncertainty Principle. According to this principle "t is impossible to determine simultaneously the exact position and momentum of a small moving particle like an electron". The postulate of Bohr, that electrons revolve in well defined orbits around the nucleus with well defined velocities is thus not attainable.
Suppose 10 J of light energy is needed by the interior of the human eye to see an object. How many photons of green light (a-550nm) are needed to generate this minimum amount of energy? Solution The energy needed to see object 10 J Photon energy used to see object 6.626x1034 x3.0x108 550x 109 3.61 x 1019 J/photon .. Number of photons- 10-17 7.7 3.61x10-19 Minimum number of photons required to see object - 28
A bulb emits light of 4500A. The bulb is rated as 150 watt and 8% of the energy is emitted as light. How many photons are emitted by the bulb per second? hc Soluion Energy of one photon- 6.626x1034 x3x10 4500x 10-10 4.42 x 1019J Energy as light emitted by bulb= 150x10- 4.42x 10-19-150x 100 n = 27.2 1018 (where n is no. of photons)
Niranjay Kumar Dwivedi
(N.K.D.Sir) is a Complete Chemistry faculty for IIT~JEE, and NEET examination. with more then 8 year of Teaching experience