Bohr’s model, quantum mechanical model of the atom

Bohr's Model of the Atom

1. Proposed by Niels Bohr in 1913.
2. Based on Rutherford’s model and Planck's quantum theory.
3. Electrons revolve around the nucleus in fixed circular paths called orbits or energy levels.
4. These energy levels are represented as n = 1, 2, 3, ... (principal quantum numbers).
5. Each orbit has a fixed amount of energy, and electrons do not radiate energy while in a stable orbit.
6. Electrons can transition between orbits by absorbing or emitting quantum energy.
7. The energy difference between two levels is given by ΔE = E₂ - E₁.
8. Explained the line spectra of hydrogen, especially the Balmer series.
9. Introduced the concept of quantized energy levels.
10. Provided stability to Rutherford’s model by addressing energy loss during electron motion.
11. Limitations: Could only explain the spectra of hydrogen and single-electron systems.
12. Failed to explain the Zeeman effect (splitting of spectral lines in a magnetic field).
13. Could not account for the fine structure of spectral lines.

Quantum Mechanical Model of the Atom

1. Developed in the 1920s, based on Schrödinger’s wave equation and Heisenberg’s uncertainty principle.
2. Describes electrons as wave-particles rather than particles in fixed orbits.
3. Electrons are found in regions of high probability called orbitals, not specific paths.
4. Each orbital is defined by a set of quantum numbers:
   • Principal quantum number (n): Indicates the energy level.
   • Azimuthal quantum number (l): Indicates the shape of the orbital (s, p, d, f).
   • Magnetic quantum number (m_l): Specifies the orientation of the orbital.
   • Spin quantum number (m_s): Describes the spin of the electron (+1/2 or -1/2).
5. The probability of finding an electron is highest near the nucleus and decreases with distance.
6. Introduced the concept of electron clouds.
7. Orbitals have specific shapes:
   • s-orbital: Spherical shape.
   • p-orbital: Dumbbell shape.
   • d- and f-orbitals: Complex shapes.
8. Explains the Pauli Exclusion Principle: No two electrons in the same atom can have identical quantum numbers.
9. Accounts for electron configuration and periodic trends.
10. Explains atomic stability and chemical bonding better than previous models.
11. Based on principles of quantum mechanics, incorporating Planck’s constant.
12. Heisenberg's Uncertainty Principle: It is impossible to simultaneously determine the exact position and momentum of an electron.
13. Accurately describes the behavior of multi-electron atoms.
14. Provides a foundation for understanding chemical reactions and molecular structures.

Key Comparisons and Features

1. Bohr’s model treats electrons as particles in circular orbits, while the quantum mechanical model describes them as wave-particles.
2. Bohr's model explains simple atomic spectra, while the quantum model explains complex atoms.
3. The quantum mechanical model is based on probability, not certainty.
4. Modern chemistry relies on the principles of the quantum mechanical model.