Textbook in PDF format This book presents quantum mechanics through a systematic progression from classical atomic models to modern quantum theory. Beginning with foundational concepts, the text develops Schrödinger's equation and its applications to fundamental quantum systems. Mathematical rigor is balanced with physical interpretation throughout. Detailed derivations of quantum mechanical principles. Comprehensive treatment of particle systems, rigid rotators, and harmonic oscillators. In-depth analysis of hydrogen atom wavefunctions and multi-electron systems. Applications to rotational, vibrational, and electronic spectroscopy. Comparative presentation of Valence Bond and Molecular Orbital theories. Ideal for advanced undergraduate and graduate students in chemistry and physics. Classical Mechanics Towards Quantum Mechanics Introduction to Quantum Mechanics Particle in a Box: Quantisation of Translational Energy Rigid Rotator and Rotational Spectra Linear Harmonic Oscillator and Vibrational Spectra “Hydrogen Atom”: Quantisation of Electronic Energy Electronic Spectroscopy Multielectron System and Approximate Methods Chemical Bonding