History and Development of Atomic Theory

Dalton and Thomson: Early Atomic Models

• Dalton: Proposed that elements consist of identical atoms (mismoelem) or different atoms (otroelem). Compounds are formed by the union of atoms, and chemical reactions involve the rearrangement of atoms.
• Thomson: Proposed a compact, positively charged sphere with embedded electrons, famously known as the "raisin pudding" model.
• Discovery of the Electron: In 1897, Thomson used a discharge tube to demonstrate the existence of negatively charged particles smaller than the atom, called electrons. He determined that the charge-to-mass ratio was constant, regardless of the cathode metal used.

Rutherford and the Discovery of the Nucleus

Rutherford, alongside Geiger and Marsden, observed that most alpha particles passed through a gold foil without deflection, while a small fraction deflected at large angles, and some bounced back toward the radioactive source.

The Rutherford Model

• The atom consists of a small, central region called the nucleus, which contains positive charges and most of the atom's mass.
• The rest of the atom is mostly empty space.

Protons and Neutrons

Rutherford's observations confirmed the presence of positive charges called protons. To explain mass differences in elements, James Chadwick bombarded beryllium with alpha particles in 1932, discovering high-energy, electrically neutral particles called neutrons.

Bohr and the Quantum Atomic Model

The Bohr Model

Based on quantum theory, the photoelectric effect, and the hydrogen atomic spectrum, Bohr proposed:

• The atom consists of a positive core surrounded by electron shells.
• Electrons describe stable circular orbits; they do not absorb or emit energy while in a stable level.
• Electrons can jump to higher levels by absorbing energy (excited atom) and emit photons when returning to lower levels.

Quantum Foundations

Max Planck argued that atoms emit or absorb energy in discrete amounts (quanta). Einstein later explained the photoelectric effect, where metals emit electrons when struck by light of a specific frequency.

The Modern Atomic Model

The current model is based on wave-particle duality and the Heisenberg uncertainty principle:

• Louis de Broglie (1924): Proposed that particles exhibit wave properties.
• Heisenberg Uncertainty Principle (1927): States that it is impossible to simultaneously know the exact position and momentum of an electron. The more precisely one variable is determined, the less precise the other becomes.