NEET Notes: Structure of Atom

Introduction to Structure of Atom

The study of atomic structure is fundamental to understanding chemistry. The atom is the smallest unit of matter that retains the properties of an element. It consists of subatomic particles such as electrons, protons, and neutrons. The concept of atomic structure has evolved over time, leading to various models explaining the behavior and arrangement of these particles. This chapter covers atomic models, quantum mechanics, and electronic configurations, which are essential for NEET aspirants.

Fundamental Particles of an Atom

Discovery of Electron

• The electron was discovered by J.J. Thomson through the cathode ray experiment.

• Cathode rays are negatively charged particles that travel from the cathode to the anode in a discharge tube.

• The charge-to-mass ratio (e/m) of an electron was determined by Thomson.

Discovery of Proton

• E. Goldstein discovered protons through the anode ray experiment (Canal rays).

• Protons are positively charged particles found in the nucleus of an atom.

Discovery of Neutron

• James Chadwick discovered neutrons, which are neutral particles found in the nucleus.

• Neutrons contribute to the atomic mass but do not affect the charge of an atom.

Atomic Models

Thomson’s Model (Plum Pudding Model)

• Proposed that an atom consists of a positively charged sphere with negatively charged electrons embedded in it.

• Failed to explain the stability of atoms and experimental observations.

Rutherford’s Nuclear Model

• Gold foil experiment: Alpha particles were bombarded on a thin gold foil.

• Observations:

  • Most particles passed through, indicating that atoms have empty space.

  • Some particles deflected, suggesting the presence of a dense, positively charged nucleus.

• Conclusion:

  • The nucleus is small, dense, and positively charged.

  • Electrons revolve around the nucleus in circular orbits.

• Limitations:

  • Did not explain electron stability or energy levels.

Bohr’s Atomic Model

• Postulates of Bohr’s Model:

  • Electrons move in fixed circular orbits (energy levels) around the nucleus.

  • Only certain allowed energy levels exist, preventing electron collapse into the nucleus.

  • Energy is absorbed or emitted when electrons jump between energy levels.

• Successes of Bohr’s Model:

  • Explained hydrogen spectrum and atomic stability.

• Limitations:

  • Could not explain spectra of complex atoms or fine structure.

Quantum Mechanical Model of Atom

Dual Nature of Matter and Light

• Wave-Particle Duality: Proposed by De Broglie, stating that particles (like electrons) exhibit both wave and particle properties.

• Heisenberg’s Uncertainty Principle: It is impossible to determine the exact position and momentum of an electron simultaneously.

Quantum Numbers

Quantum numbers describe the position and energy of electrons in an atom.

• Principal Quantum Number (n): Indicates the energy level (shell) of an electron.

• Azimuthal Quantum Number (l): Defines the shape of the orbital (s, p, d, f).

• Magnetic Quantum Number (m): Describes the orientation of the orbital in space.

• Spin Quantum Number (s): Indicates the spin of an electron (+½ or -½).

Shapes of Orbitals

• s-Orbital: Spherical shape.

• p-Orbital: Dumbbell shape.

• d-Orbital: Complex cloverleaf shape.

• f-Orbital: More complex shapes.

Electronic Configuration of Elements

Aufbau Principle

• Electrons fill orbitals in increasing energy order.

• The filling order follows 1s < 2s < 2p < 3s < 3p < 4s < 3d < 4p and so on.

Pauli’s Exclusion Principle

• No two electrons in an atom can have the same set of four quantum numbers.

Hund’s Rule of Maximum Multiplicity

• Electrons fill degenerate orbitals (same energy level) singly before pairing.

Stability of Half-Filled and Fully Filled Orbitals

• Atoms tend to achieve half-filled (e.g., Cr: 3d⁵) or fully filled (e.g., Cu: 3d¹⁰) orbitals due to extra stability.

Atomic Spectra and Emission of Light

Hydrogen Spectrum

• The line spectrum of hydrogen consists of different series:

  • Lyman Series: Ultraviolet region.

  • Balmer Series: Visible region.

  • Paschen, Brackett, and Pfund Series: Infrared region.

• The energy of emitted photons corresponds to transitions between different energy levels.

Photoelectric Effect

• Einstein’s Photoelectric Equation: Light behaves as particles (photons), and energy is absorbed in discrete quanta.

• Electrons are ejected from a metal surface when light of suitable frequency is incident on it.

Isotopes, Isobars, and Isotones

Isotopes

• Atoms of the same element with the same atomic number but different mass numbers (e.g., ¹H, ²H, ³H).

Isobars

• Atoms with the same mass number but different atomic numbers (e.g., ⁴⁰Ar and ⁴⁰K).

Isotones

• Atoms with the same number of neutrons but different protons (e.g., ⁴⁰Ca and ³⁹K).

Applications of Atomic Structure in Chemistry and Technology

• Medical Imaging: Isotopes like Technetium-99m are used in medical diagnostics.

• Nuclear Energy: Atomic fission reactions are used in nuclear power plants.

• Spectroscopy: Atomic spectra are used in chemical analysis and astrophysics.

Conclusion

The structure of an atom is essential for understanding chemical bonding, periodic trends, and the behavior of elements. The quantum mechanical model provides a more accurate representation of electron distribution than previous models. A strong grasp of these concepts is crucial for NEET aspirants, as it forms the basis for various topics in chemistry.