Bohr’s Model Postulates of an Atom

Negative charged atom’s electrons orbit the +ve charged nucleus in a fixed circular path known as orbits. A circular path is called an orbit or shell, and circular orbits are known as orbital shells. In this model, the electron moves from a higher to a lower energy state. Such circular orbits are called orbital shells because each orbit has defined energy. Quantum numbers, which are integers, are used to denote the level of energy of these orbits. The nucleus is at the beginning of this quantum range of numbers, with n=1 having the lowest level of energy. The orbits n=one, two… are assigned shells K, L,  and an electron is said to be in the initial state when it reaches the lowest energy state.

When an electron in an atom moves from a lower to a higher energy level, it acquires energy, and when it moves from an energy level to a lower stage, it loses energy. 

Defects of Rutherford’s model

New-Zealand born physicist Ernest Rutherford formulated the Rutherford model to describe an atom. According to this model of Rutherford, an atom contains a +ve nucleus with electrons travelling in circular orbits around it. James Clerk Maxwell, on the contrary, demonstrated that when an electron is accelerated, it emits radiation and loses energy. As a result, its orbit should become narrower and narrower, eventually collapsing into the nucleus in a spiral pattern. As a result, the atom would come crashing down, and Rutherford’s model would fail to address atomic stability.

Another flaw in Rutherford’s model is that it reveals nothing at all about the electronic structure of atoms, such as how electrons are arranged all around the nucleus and what their energies are. 

Bohr’s model of an atom postulates

To address the model of Rutherford’s flaws, Niels Bohr proposed a version based on the quantum theory of radiation. The following are the most critical postulates: 

1. Only a few circular paths known as orbits permit electrons to orbit the nucleus. These orbits are referred to as energy shells, energy states, or quantum states since they are associated with specific energies. These are numbered one, two, etc., and are labelled K, L, and so forth (beginning with the nucleus). A circular path is called an orbit or shell, and circular orbits are known as orbital shells. In this model, an electron moves from a higher to a lower energy state.
2. As long as an electron is in a certain orbit, it does not gain and loses energy. This implies that an electron’s energy in a certain path stays unchanged. As a result, these orbits are frequently called stationary levels or states.
3. Only orbits where the electron’s angular momentum is a multiple of a whole number of h/2pi are allowed. The electron’s angular momentum travelling in a circular orbit is equal to mnr, where m is the electron’s mass and n is the angular momentum, and mvr is a multiple of the whole number of h/2pi.
   The electron’s angular velocity in an atom is quantised, in other words.
4. When an electron goes from one state to another, it is said to be moving from one stationary state to another. It absorbs or emits radiation with a specific frequency, resulting in a spectral line with that frequency that relies on the final and initial states. When an electron goes to a lower energy state, it emits energy in the same way that radiation does.

Limitation of Bohr’s theory

When an electron returns to a lower energy state, it emits a relatively similar quantity of energy as radiation.

1. The existence of different lines in the H-spectrum was explained by Bohr Theory. However, it is expected that only a few lines exist. This had been noticed at the time. With the development of improved instruments and procedures, it was discovered that the spectral line that had been assumed to be a single line was a group of multiple lines that were quite similar near (called fine spectrum). The Balmer series’ H spectral band, for example, is made up of numerous lines that are relatively close together. 

2. As a result, the existence of many lines means that with every quantum number n, several levels of energy are near together. This would necessitate the creation of novel quantum numbers.  

3. He could not give acceptable reasoning for the supposition that the electron can cycle only in certain orbits in which the momentum of the angular electron (mvr) is a multiple of a whole number of h/2𝜋. He could not give any reason for using the concept of quantisation of angular momentum, which he invented randomly. 

4. An electron in an atom, according to Bohr, is placed from the nucleus at a specific length and revolves around it with a constant velocity, i.e. it has a fixed amount of momentum. This goes against the Uncertainty Principle of Heisenberg, which states that determining a particle’s location and momentum concurrently with confidence is impossible.

Conclusion

The atom has a large core nucleus in the centre, and neutrons and protons are contained in this atom. The nucleus is relatively modest in size. In a bit, the electron moves in distinct orbitals. Stable orbits, also known as non-radiating or stationary orbits, are just such orbits. The centrifugal force of the travelling electron The force of attraction between oppositely charged ions and the electron is directly proportional. The force of attraction between oppositely charged ions and the electron is directly proportional.