bohr,s atomic theory
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In 1913 Bohr proposed his quantized shell model of the atom to explain how electrons can have stable orbits around the nucleus. The motion of the electrons in the Rutherford model was unstable because, according to classical mechanics and electromagnetic theory, any charged particle moving on a curved path emits electromagnetic radiation; thus, the electrons would lose energy and spiral into the nucleus. To remedy the stability problem, Bohr modified the Rutherford model by requiring that the electrons move in orbits of fixed size and energy. The energy of an electron depends on the size of the orbit and is lower for smaller orbits. Radiation can occur only when the electron jumps from one orbit to another. The atom will be completely stable in the state with the smallest orbit, since there is no orbit of lower energy into which the electron can jump.
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A Danish physicist named Neil Bohr in 1913 proposed the Bohr atomic model. He modified the problems and limitations associated with Rutherford’s model of an atom. Earlier in Rutherford Model, Rutherford explained in an atom a nucleus is positively charged and is surrounded by electrons (negatively charged part ).
The electrons move around in a predictable path called orbits. Bohr modified Rutherford’s model where he explained that electrons move around in fixed orbital shells. Furthermore, he explained that each orbital shell has fixed energy levels. Therefore, Rutherford basically explained a nucleus of an atom whereas Bohr took the model one step ahead. He explained about electrons and the different energy levels associated with it.
According to Bohr Atomic model, a small positively charged nucleus is surrounded by revolving negatively charged electrons in fixed orbits. He concluded that electron will have more energy if it is located away from the nucleus whereas the electrons will have less energy if it located near the nucleus.
Postulates of the Bohr Atomic Model
Electrons revolve around the nucleus in a fixed circular path termed “orbits” or “shells” or “energy level.”
The orbits are termed as “stationary orbit.”
Every circular orbit will have a certain amount of fixed energy and these circular orbits were termed orbital shells. The electrons will not radiate energy as long as they continue to revolve around the nucleus in the fixed orbital shells.
The different energy levels are denoted by integers such as n=1 or n=2 or n=3 and so on. These are called as quantum numbers. The range of quantum number may vary and begin from the lowest energy level (nucleus side n=1) to highest energy level. Learn the concept of an Atomic number here.
The different energy levels or orbits are represented in two ways such as 1, 2, 3, 4… or K, L, M, N….. shells. The lowest energy level of the electron is called the ground state. Learn the concept of Valency here in detail.
The change in energy occurs when the electrons jump from one energy level to other. In an atom, the electrons move from lower to higher energy level by acquiring the required energy. However, when an electron loses energy it moves from higher to lower energy level.
Therefore,
1st orbit (energy level) is represented as K shell and it can hold up to 2 electrons.
2nd orbit (energy level) is represented as L shell and it can hold up to 8 electrons.
3rd orbit (energy level) is represented as M shell and it can contain up to 18 electrons.
4th orbit (energy level) is represented as N Shell and it can contain maximum 32 electrons.
The orbits continue to increase in a similar manner.
Distribution of Electrons in Orbits or Shells:
Electronic distribution of various orbits or energy levels can be calculated by the formula 2n2. Here, ‘n’ denotes the number of orbits.
The number of electrons in K shell (1st orbit) can be calculated by 2n2= 2 x 12 = 2. Thus, maximum number of electrons in 1st orbit = 2
Similarly, The number of electrons in L shell (2nd orbit)= 2 x 22 = 8. Thus, maximum number of electrons in 2nd orbit = 8
We can determine the maximum number of electrons in a similar way.
Read about Thomson’s Model of an Atom, the very first model of an Atom by J.J. Thomsons.
Limitations of Bohr’s Model of an Atom:
Bohr atomic model had few limitations. They are:
Failure to explain Zeeman Effect (how atomic spectra are affected by magnetic fields).
It contradicts Heisenberg Uncertainty Principle.
Unable to explain how to determine the spectra of larger atoms.