Neil Bohr’s Atomic Model is an improvement of Rutherford’s Atomic Model and was presented by both Niels Bohr and Ernest Rutherford in 1913. This post will discuss about what is Neil Bohr’s Atomic Model, how it different from Rutherford’s Atomic Model, its three postulates and limitations.
What is Neil Bohr’s Atomic Model
According to Niels Bohr, his Atomic Model explained the existence of an Atom which consists of a small, dense Nucleus which is surrounded by orbiting electrons much like the structure of the Solar System, where the attraction between the particles is due to electrostatic force rather than gravity.
Fig. 1 – Introduction to Bohr’s Atomic Model
The improvement done to Rutherford’s Model is mainly a quantum physical interpretation of the model. The success of the Bohr’s Atomic Model lies in explaining the Rydberg formula for the spectral emission lines of atomic hydrogen. Rydberg’s observation can be seen in Bohr’s theory as the description of the energies of transitions or quantum jumps between orbital energy levels.
Fig. 2 – Bohr’s Atomic Model
Rutherford deduced that Atom comprised of a diffuse cloud of negatively charged electrons that surrounded a tiny, dense, positively charged nucleus in the early 20th century. Rutherford imagined a planetary model of an atom but it had a mechanical drawback. When the laws of classical mechanics are applied, the electron will release an electromagnetic radiation while orbiting a nucleus. So the electron would lose its energy, and rapidly spiral inwards and collapse into the Nucleus on a timescale of approximately 16 picoseconds.
The Rutherford Model thus becomes inaccurate as it projects that all Atoms are unstable. Also, if the electrons spiral inwards, the emission would increase the frequency rapidly as the orbit would get smaller and faster. This would result in a continuous streak in frequency of the electromagnetic radiation. To overcome the drawbacks in the Rutherford Model, Bohr proposed his Atomic Model and put forward three Postulates.
Fig. 3 – Image of Neil Bohr
Postulates of Bohr’s Atomic Model
The three Postulates stated by Neil Bohr are:
- The electron can revolve in certain stable orbits around the Nucleus without radiating any energy, in contrast to what classical electromagnetism suggests. The stable orbits are called stationary orbits. They are gained at certain distinct distances from the nucleus. The electrons cannot have any other orbit in between the distinct ones.
- An electron can only gain or lose energy by jumping from one entitled orbit to another by absorbing or emitting electromagnetic radiation with a frequency ‘ν’ which is determined by the energy difference of the shells according to the Planck’s relation:
∆E = E2-E1 = hv
Where h = Planck’s Constant and E = Energy
- Bohr’s Atomic Model was based upon Planck’s quantum theory of radiation. The stationary orbits are gained at distances when the angular momentum of the revolving electron is an integral multiple of the reduced Planck’s constant:
m is mass of the electron
v is the velocity of the electron
r is the radius of the electron,
n = 1, 2, 3, and so on, is called the principal quantum number, and h is the Planck’s constant.
Fig. 4 – Bohr’s Atomic Model
The lowest value of n is 1; which gives the smallest possible orbital radius of 0.0529 nm known as the Bohr radius. When an electron is in the lowest orbit, it cannot get any closer to the Proton. Establishing from the angular momentum quantum rule, Bohr could calculate the energies of the allowed orbits of the Hydrogen atom and some other hydrogen-like atoms and ions. The orbits are associated with definite energies and are also known as energy levels or energy shells. In these orbits, the acceleration of the electron does not result in radiation or energy loss.
Limitations of Bohr Atomic Model
Few limitations are listed below:
- This model cannot explain much of the spectra of larger atoms. It can only predict the K-alpha or L-alpha X-ray emission spectra for larger atoms only if two additional ad hoc assumptions are made. Emission spectra of atoms with one outer-shell electron, such as atoms of the lithium group, can only be roughly predicted. If the empirical electron–nuclear screening factors are known for many atoms, many other spectral lines can be inferred from the information in similar atoms of different elements using the Ritz–Rydberg combination principles. All these methods actually use Bohr’s Newtonian energy-potential model of the atom.
- Bohr’s Atomic Model has difficulty with the relative intensities of spectral lines. Though in certain simpler cases, Bohr’s formula or the modifications of it can provide reasonable estimates, for example, the calculations of Stark effect of Kramers Henneberger (KH) atoms.
- It cannot explain the existence of fine or hyperfine structure in spectral lines, which are the result of a variety of relativistic and subtle effects, and also complications from the electron spin.
- The model cannot explain the Zeeman effect. The Zeeman effect refers to the changes in spectral lines caused by external magnetic fields. These are also a result of more complicated quantum principles interacting with electron spin and orbital magnetic fields.
- It violates the uncertainty principle which considers electrons to have known orbits and locations, both these parameters which cannot be measured simultaneously.
- Bohr’s Atomic Model has difficulty in explaining Doublets and Triplets which appear in the spectra of certain atoms and are very close pairs of orbits. Bohr’s Atomic Model cannot explain why certain energy levels could be so close together.
- Bohr’s Atomic Model cannot predict why atoms with many electrons don’t have energy levels. It also doesn’t work for neutral atoms like helium.