NCERT Solutions for Class 11 Chemistry Bohr’s Model Of Atom
Chemistry notes for Class 11: Niel’s Bohr proposed the Bohr Model of the Atom in 1913. As the Bohr model is a modification of the earlier Rutherford Model, some people call Bohr’s Model the Rutherford-Bohr Model. The Bohr Model is a planetary model in which the negatively-charged electrons orbit a small, positively-charged nucleus similar to the planets orbiting the Sun (except that the orbits are not planar). The gravitational force of the solar system is mathematically akin to the Coulomb (electrical) force between the positively-charged nucleus and the negatively-charged electrons. Bohr depicted the atom as a tiny, spherical body which consists of the nucleus at the center and negatively charged electrons revolving around the nucleus in a certain path known as orbit.
In order to explain the stability of an atom, Neils Bohr gave a new arrangement of electrons in the atom in 1913. According to Bohr, the electrons could revolve around the nucleus in only certain orbits called energy levels, each having a different radius. When an electron revolves in a particular orbit or energy level around the nucleus, it does not radiate energy (lose energy) even though it has accelerated motion around the nucleus.
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Bohr Atomic Model
- An atom is made up of three particles – electrons, protons, and Electrons have a negative charge and protons have a positive charge whereas neutrons have no charge. They are neutral. Due to the presence of an equal number of negative electrons and positive protons, the atom as a whole is electrically neutral.
- The protons and electrons are located in a small nucleus at the center of the atom. Due to the presence of protons, the nucleus is positively charged.
- The electrons revolve rapidly around the nucleus in fixed circular paths called energy levels or shells. The ‘energy levels’ or ‘shells’ or ‘orbits’ are represented in two ways: either by the numbers 1, 2, 3, 4, 5 and 6 or by letters K, L, M, N, O, and The energy levels are counted from the center outwards.
Arrangement of orbits around the nucleus
- Each energy level is associated with a fixed amount of energy. The energy of an electron in a certain orbit remains constant. As long as it remains in that orbit, it neither emits nor absorbs energy. These are termed stationary states or main energy states.
- The shell nearest to the nucleus has minimum energy and the shell farthest from the nucleus has maximum energy.
- Bohr proposed that the angular momentum of an electron is quantized. Thus, the motion of an electron is restricted to those orbits where its angular momentum is an integral multiple of h/2π, where h is Planck’s constant.
- Thus we have the relationship mvr = nh/2π, where m is mass of the electron, v is the velocity of electron of said orbit; r is the radius of that orbit, n is a simple integer.
- The stationary states or allowed energy levels are only those where n = 1, 2, 3,…… This is called the Bohr quantum condition.
- The energy of an electron changes only when it moves from one orbit to another. An electronic transition from an inner orbit to outer orbit involves absorption of energy. Similarly, when an electron jumps from an outer orbit to inner orbit it releases energy, which is equal to the difference between the two energy levels.
- The energy thus released in the form of a radiation of a certain frequency appears in the form a line in the atomic spectrum. If the energy of an electron in the outer orbit (n2) is E2 and energy of an electron in the inner orbit (n1) is E1 then E2 – E1 = ΔE = hν.
- The value of n could be small integers 1, 2, 3 and these correspond to the first, second, third, and so on. Quantum states are shells for the electron; n is termed as a principal quantum number.
- This model of the atom was able to explain the stability of the atom. It also explained the phenomenon of atomic spectra and ionization of gases.
Based on this theory, Bohr calculated the radii of the various orbits and the energies associated with the electrons present in those shells.
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Problems with the Bohr model
- It violates the Heisenberg Uncertainty Principle because it considers electrons to have both a known radius and orbit.
- The Bohr Model provides an incorrect value for the ground state orbital angular momentum.
- It makes poor predictions regarding the spectra of larger atoms.
- It does not predict the relative intensities of spectral lines.
- The Bohr Model does not explain the fine structure and hyperfine structure in spectral lines.
- It does not explain the Zeeman Effect ((effect of magnetic field on the spectra of atoms).
Read more topics of NCERT Chemistry Class 11
Chemical Bonding and Molecular Structure
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