NEILS BOHR MODEL OF THE ATOM?
Answers
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.
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.
Bohr:
■accepted Rutherford’s nucleus model of atom and Planck’s quantum theory.
■proposed an atomic model to explain the spectra emitted by hydrogen atoms.
Called planetary model of the atom.
Based on four postulates.
■Postulates:
★NUCLEAR CONCEPT:
An atom consists of small and massive central core, called as the nucleus, around which planetary electrons revolve. The centripetal force required for their revolution around the atom’s nucleus is provided by the electrostatic force of attraction between the nucleus and the electron.
★QUANTUM CONDITION:
Of all the possible circular orbits allowed by the classical theory, the electrons are allowed to revolve only in those orbits in which, the angular momentum of the electron is an integral multiple of h/2pi. This is Bohr’s famous quantum condition.
★STATIONARY ORBITS:
An electron does not radiate energy, while revolving in a permissible orbits. These permissible, non - radiating orbits are called as stationary orbits.
★FREQUENCY CONDITION:
An electron can absorb or emit radiation in the form of discrete energy photons only when it jumps from a lower energy orbit to a higher energy orbit or from a higher energy orbit to a lower energy orbit, respectively. This is Bohr’s famous frequency condition.
