Atomic Structure 1.1

1.1 Introduction

The atomic and molecular structure forms the backbone of chemistry, providing insight into how atoms bond and interact to form molecules.

1.2 Electromagnetic Radiation

Electromagnetic radiation is energy transmitted in the form of waves or particles across space or through a medium. It encompasses a broad range of wavelengths, from radio waves to gamma rays, but the most important in chemistry are visible light, ultraviolet (UV), and infrared (IR) radiation.

The energy \(E\) of electromagnetic radiation is given by Planck's relation:

Failure of Classical Mechanics

Till the end of the 19^th century, Classical or Newtonian Mechanics was considered to be the only right and undisputed theoretical science. But soon, some new experimental phenomena were observed that could not be explained by Classical Mechanics. This includes:

• Black Body Radiation
• Photoelectric Effect

Classical mechanics gives the laws of motion of macroscopic objects, whereas quantum mechanics gives the laws of motion of microscopic objects.

Photoelectric Effect

The ejection of electrons from the surface of a metal by radiation is called the photoelectric effect.

Classically, electron ejection from the metal’s surface was thought to occur because of the oscillation of electrons with light, eventually breaking away from the surface with a kinetic energy dependent on the intensity of the incident radiation.

However, experimentally it was found that the kinetic energy of ejected electrons was independent of radiation intensity.

In 1905, Albert Einstein explained the photoelectric effect using Planck’s relation between energy and frequency. He proposed that light of frequency \(\nu\) consists of particles (photons) with energy \(h\nu\).

This explained the observed dependence of electron energy on light frequency, giving light a particle character.

Key Conclusions

1. Energy is quantized — transferable only in discrete packets called quanta.
2. Light exhibits particle-like behavior.

Wave-Particle Dual Behavior

In 1924, Louis de Broglie proposed that if radiation has dual behavior, matter should too. He suggested particles have wave-like properties with wavelength:

For macroscopic objects, the wavelengths are too small to observe. For example, a \(100\ \mathrm{kg}\) person walking at \(5\ \mathrm{m/s}\) has \(\lambda \approx 1.32\times 10^{-36}\ \mathrm{m}\).

Wave-like behavior of matter was confirmed experimentally by Davisson and Germer, who observed electron diffraction off a crystal lattice.

Heisenberg Uncertainty Principle

In 1926, inspired by wave mechanics, Heisenberg proposed the uncertainty principle:

This states that the position and momentum of a particle cannot be simultaneously determined with arbitrary precision.

Shorter wavelength light improves position measurement but increases momentum uncertainty, and vice versa.

Summary

• Quantum mechanics governs microscopic matter.
• Quantum chemistry explains structure, bonding, and reactivity.
• Quantization: physical quantities can take only discrete values.
• Quantum mechanics explains phenomena (e.g., blackbody radiation, photoelectric effect) that classical mechanics cannot.