Bohr’s Atomic Model
A Danish physicist called Neil Bohr 1913 proposed the Bohr atomic model.
The Bohr model was an improvement on the earlier cubic design (1902), the plum-pudding model (1904), the Saturnian model (1904), and the Rutherford model (1911). Since the Bohr design is a quantum-physics-based modification of the Rutherford model, numerous sources combine the two: the Rutherford– Bohr model.
Although it challenged the knowledge of classical physics, the model’s success lay in describing the Rydberg formula for the spectral emission lines of atomic hydrogen. While the Rydberg formula had been understood experimentally, it did not acquire a theoretical underpinning till the Bohr model was presented.
Not just did the Bohr design explain the factor for the structure of the Rydberg formula, it likewise offered a justification for its empirical results in terms of basic physical constants.
Early planetary designs of the atom suffered from a defect: they had electrons spinning in orbit around a nucleus– a charged particle in an electrical field. There was no accounting for the truth that the electron would spiral into the nucleus. In terms of electron emission, this would represent a continuum of frequencies being released given that, as the electron moved more closed to the nucleus, it would move quickly and would release various frequencies than those experimentally observed.
These planetary models eventually forecasted all atoms to be unsteady due to the orbital decay. The Bohr theory solved this problem and correctly described the experimentally obtained Rydberg formula for emission lines.
Postulates of Bohr’s Atomic Model
- Electrons revolve around the nucleus in a fixed circular path described as “orbits” or “shells” or “energy level.”
- The orbits are termed as “stationary orbit.”
- Every circular orbit will have a particular amount of fixed energy and these circular orbits were called orbital shells. The electrons will not radiate energy as long as they continue to move around the nucleus in the fixed orbital shells.
- The various energy levels are represented by integers such as n= 1 or n= 2 or n= 3 and so on. These are called quantum numbers. The range of quantum numbers might differ and begin from the lowest energy level (nucleus side n= 1) to a higher energy level.
- The various energy levels or orbits are represented in 2 methods such as 1, 2, 3, 4 … or K, L, M, N. … shells. The lower energy level of the electron is called the ground state.
- The change in energy takes place when the electrons jump from one energy level to another. In an atom, the electrons move from lower to higher energy levels by getting the needed energy. Nevertheless, when an electron loses energy it moves from higher to lower energy level.
- This change in energy, ΔE is given by following Planck’s equation
Δ E = E2–E1 = h v
Where h is Planck’s constant equal to 6.63 x 10 -34Js and v is the frequency of light.
- An electron can revolve only in orbits of a fixed angular moment mvr.
For that reason,
- 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 consist of up to 18 electrons.
- 4th orbit (energy level) is represented as N Shell and it can contain optimal 32 electrons.
Limitations of Bohr’s Model of an Atom
Bohr’s model of an atom stopped working to describe the Zeeman Effect (effect of magnetic field on the spectra of atoms).
It likewise stopped working to explain the Stark result (impact of electric field on the spectra of atoms).
It violates the Heisenberg Uncertainty Principle.
It could not discuss the spectra obtained from larger size atoms.
MCQs:
- What was the improvement made by Bohr’s atomic model over earlier atomic models?
- A) Introduction of electron shells
- B) Discovery of the nucleus
- C) Description of electron spin
- D) Identification of atomic orbitals
- Answer: A) Introduction of electron shells
- What is the significance of Bohr’s atomic model regarding the Rydberg formula?
- A) It disproved the Rydberg formula
- B) It provided a theoretical basis for the Rydberg formula
- C) It had no relation to the Rydberg formula
- D) It led to the discovery of the Rydberg constant
- Answer: B) It provided a theoretical basis for the Rydberg formula
- Which of the following correctly describes the motion of electrons in Bohr’s model?
- A) Spiral motion towards the nucleus
- B) Circular motion in fixed orbits
- C) Random motion within the nucleus
- D) Linear motion away from the nucleus
- Answer: B) Circular motion in fixed orbits
- What term is used to describe the fixed circular paths in which electrons revolve around the nucleus in Bohr’s model?
- A) Orbits
- B) Shells
- C) Levels
- D) Stations
- Answer: A) Orbits
- According to Bohr’s model, when do electrons emit or absorb energy?
- A) When they are stationary
- B) When they move closer to the nucleus
- C) When they jump between energy levels
- D) When they collide with other electrons
- Answer: C) When they jump between energy levels
- What term is used to represent the various energy levels or orbits in Bohr’s model?
- A) Quantum shells
- B) Stationary shells
- C) Orbital shells
- D) Quantum orbits
- Answer: C) Orbital shells
- Which equation describes the change in energy of electrons when they transition between energy levels in Bohr’s model?
- A) E = mc^2
- B) E = hf
- C) ΔE = E2–E1 = hv
- D) F = ma
- Answer: C) ΔE = E2–E1 = hv
- How many electrons can the K shell hold in Bohr’s atomic model?
- A) 2
- B) 8
- C) 18
- D) 32
- Answer: A) 2
- What is represented by the letter ‘h’ in Planck’s equation, ΔE = hv?
- A) Electron charge
- B) Speed of light
- C) Planck’s constant
- D) Frequency of light
- Answer: C) Planck’s constant
- What is the primary limitation of Bohr’s model regarding the Zeeman Effect?
- A) It failed to explain the Stark effect
- B) It violated the Heisenberg Uncertainty Principle
- C) It couldn’t account for the effect of magnetic fields on atomic spectra
- D) It couldn’t explain spectra from larger size atoms
- Answer: C) It couldn’t account for the effect of magnetic fields on atomic spectra
- Which principle does Bohr’s atomic model violate?
- A) Pauli Exclusion Principle
- B) Aufbau Principle
- C) Heisenberg Uncertainty Principle
- D) Hund’s Rule
- Answer: C) Heisenberg Uncertainty Principle
- How many electrons can the N shell hold in Bohr’s atomic model?
- A) 2
- B) 8
- C) 18
- D) 32
- Answer: D) 32
- What is the maximum number of electrons that can occupy the M shell in Bohr’s model?
- A) 2
- B) 8
- C) 18
- D) 32
- Answer: C) 18
- What term describes the circular orbits in which electrons revolve around the nucleus in Bohr’s atomic model?
- A) Quantum orbits
- B) Electron orbits
- C) Orbital shells
- D) Stationary orbits
- Answer: D) Stationary orbits
- Which of the following statements is true regarding the motion of electrons in Bohr’s atomic model?
- A) Electrons move randomly within the nucleus
- B) Electrons spiral towards the nucleus
- C) Electrons move in circular orbits with fixed energy levels
- D) Electrons move linearly away from the nucleus
- Answer: C) Electrons move in circular orbits with fixed energy levels
- What concept introduced by Bohr’s model provided a theoretical basis for the Rydberg formula?
- A) Electron shells
- B) Electron spin
- C) Orbital motion
- D) Quantum numbers
- Answer: A) Electron shells
Summary
In conclusion, Bohr’s Atomic Model, proposed by Danish physicist Niels Bohr in 1913, revolutionized our understanding of atomic structure and spectral emissions. This model, an improvement over earlier atomic models, successfully described the spectral emission lines of hydrogen, providing a theoretical basis for the empirically derived Rydberg formula.
Key points of Bohr’s model include:
- Electrons orbit the nucleus in fixed circular paths termed as orbits or shells.
- Each orbit has a specific energy level, and electrons do not radiate energy as long as they remain in these fixed orbits.
- Energy levels are represented by quantum numbers, and transitions between these levels result in the emission or absorption of energy, as described by Planck’s equation.
- The model introduced the concept of stationary orbits and the limitation on the angular momentum of electrons.
Despite its successes, Bohr’s model had limitations, including its inability to explain phenomena like the Zeeman Effect and the Stark effect. It also violated the Heisenberg Uncertainty Principle and couldn’t fully account for the spectra of larger atoms.
Overall, Bohr’s Atomic Model laid the groundwork for quantum mechanics and greatly advanced our understanding of atomic physics and spectral analysis.