Bohr’s Model of an Atom

Neil Bohr proposed the Bohr model and was based on the modification of Rutherford’s model of an atom. Rutherford’s model introduced a nuclear model of an atom in which it was explained that the nucleus which is positively charged is surrounded by negatively charged particles called electrons. Bohr modified it where it was explained that electrons move in fixed orbitals and not in between. He went on to say that each shell has a specific energy level. As a result, Rutherford explained an atom’s nucleus, and Bohr changed it into electrons and their energy levels.

Rutherford’s Model of an atom

Rutherford aimed high-energy alpha-particle streams from a radioactive source towards a thin sheet of gold in his experiment (100 nm thickness). To analyse the deflection generated by the alpha-particles, he draped a fluorescent zinc sulphide screen around the thin gold foil. Rutherford’s observations contradicted Thomson’s atomic model in some ways.

Observations of Rutherford’s Alpha Scattering Experiment:

1. Because a large portion of the alpha-particles fired at the gold sheet passed through it without being deflected, the majority of the space in one atom is vacant.
2. The gold sheet deflected some of the alpha particles at minor angles, causing the positive charge in each atom to be unevenly distributed. The positive charge in an atom is concentrated in a small area of the atom.
3. Only a few alpha particles were deflected back, implying that only a few alpha particles had nearly 180 degrees deflection angles. As a result, the positively charged particles in an atom occupy a very small volume in comparison to the total volume of the atom.

Limitations of Rutherford’s Model: 

• According to Rutherford, electrons orbit the nucleus in set trajectories termed orbits. According to Maxwell, accelerated charged particles emit electromagnetic radiation, so an electron revolving around the nucleus should do the same. This radiation would transmit energy from the electron’s motion, but at the expense of orbital shrinkage. The electrons would eventually collapse in the nucleus. According to calculations, an electron would disintegrate in the nucleus in less than 10-8 seconds using the Rutherford model. As a result, the Rutherford model contradicted Maxwell’s theory and was unable to explain atom stability.
• One of the problems in Rutherford’s model was that he ignored the electron arrangement in an atom, leaving his theory incomplete.
• Despite the fact that the early atomic models were erroneous and failed to explain certain experimental results, they served as the foundation for future quantum mechanics breakthroughs.

Bohr’s Model of an atom

Bohr’s model consists of a small nucleus (positively charged) surrounded by negative electrons moving around the nucleus in orbits where he found out that an electron located away from the nucleus has more energy as compared to electrons close to the nucleus.

Bohr’s model of an atom can be described where an atom is made up of three sub-atomic particles – protons, electrons, neutrons. Proton is a positively charged particle that is a fundamental constituent of all atomic nuclei. It is the lightest and most stable and has a charge which is equal in magnitude to that of the electron. An electron is a particle that is a fundamental constituent of matter that exists independently or as the component outside the nucleus of an atom. A neutron is an elementary particle that has no charge and which has a mass slightly greater than that of a proton. Protons and neutrons are housed in a tiny nucleus at the atom’s core.

The electrons spin rapidly around the nucleus in circular routes known as energy levels or shells. The number of electrons that each shell can hold is limited. The formula is, 2n²  where n is the orbit number or energy level index.

Each energy level is associated with a fixed amount of energy and there is no change in the energy of the electrons as long as they keep revolving in the same energy level, and the atom remains stable.

Postulates of Bohr’s Model of an Atom

1. In an atom, electrons that are negatively charged revolve around the positively charged nucleus in a circular path called orbits or shells.
2. Each orbit or shell has fixed energy and is called an orbital shell.
3. Energy levels are represented by an integer (n=1, 2, 3…). These integers are also called quantum number that starts from nucleus with n=1 which has the lowest energy level. An electron is said to be in the ground state when it reaches the lowest energy level.
4. When electrons gain energy, they move from a lower energy level to a higher energy. When they lose energy, they move from a higher energy level to lower energy level.

Limitations of Bohr’s Model of Atom

1. It failed to explain the Zeeman Effect. In the presence of a static magnetic field, the Zeeman effect causes a spectral line to break into numerous components. It’s similar to the Stark effect.
2. It also failed to explain the Stark effect. The Stark effect is the splitting of a spectral line into several components in the presence of an electric field.
3. It violated the Heisenberg Uncertainty Principle and couldn’t explain the spectra of bigger atoms. This concept asserts that an object’s position and velocity cannot be determined precisely at the same time. In fact, in nature, the concepts of absolute position and exact velocity have no relevance.

Sample Questions 

Question 1: What was the relation between Bohr’s model and Rutherford’s model?

Solution:

Bohr’s model was formed by the modification of Rutherford’s model of an atom that introduced nuclear model of an atom wherein it was explained that nucleus which is positively charged is surrounded by negatively charged electrons. It was modified by Bohr where he explained that electrons move in fixed orbitals and not anywhere in between. It was also explained that each shell has a fixed energy level. 

Question 2: What is Bohr’s model of an atom?

Solution:

Bohr’s model consists of a small nucleus (positively charged) surrounded by negative electrons moving around the nucleus in orbits where he found out that an electron located away from the nucleus has more energy as compared to electrons close to the nucleus.

Question 3: How do electrons move in Bohr’s model? 

Solution:

Electrons in atoms travel around a central nucleus in circular orbits and can only orbit stably at a fixed set of distances from the nucleus in circular orbits that are related to energies.

Question 4: How did Sommerfeld modify Bohr’s theory? 

Solution:

Sommerfeld model suggested that electrons move around a nucleus in elliptical orbits instead of circular orbits. The Bohr – Sommerfeld model was incoherent that contributed to many paradoxes.

Question 5: What were the limitations of the Rutherford’s model of an atom? 

Solution:

According to Rutherford, electrons orbit the nucleus in set trajectories termed orbits. According to Maxwell, accelerated charged particles emit electromagnetic radiation, so an electron revolving around the nucleus should do the same. This radiation would transmit energy from the electron’s motion, but at the expense of orbital shrinkage. The electrons would eventually collapse in the nucleus. According to calculations, an electron would disintegrate in the nucleus in less than 10-8 seconds using the Rutherford model. As a result, the Rutherford model contradicted Maxwell’s theory and was unable to explain atom stability.

One of the problems in Rutherford’s model was that he ignored the electron arrangement in an atom, leaving his theory incomplete.

Despite the fact that the early atomic models were erroneous and failed to explain certain experimental results, they served as the foundation for future quantum mechanics breakthroughs.