Atomic Theory: History and Development

• Atomic theory explains that matter consists of discrete units called atoms.
• While the idea originated in ancient Greece, the modern atomic theory began its development in the early 19th century.

Ancient Philosophers

• Leucippus and Democritus, Greek philosophers from the 5th century BCE, initially proposed the concept of atoms.
• Democritus named these indivisible particles "atomos," which means "uncuttable".
• The initial atomic theories lacked experimental support and were mainly philosophical.
• Aristotle's belief in earth, air, fire, and water as the fundamental elements was a competing idea.
• The widespread acceptance of Aristotle's views slowed down the progress of atomic theory for centuries.

John Dalton and the Modern Atomic Theory

• John Dalton, an English chemist and physicist, is credited with formulating the first modern atomic theory in the early 19th century.
• Dalton's theory used experimental evidence to explain chemical phenomena scientifically.
• Dalton's atomic theory is based on four postulates:
  • Matter consists of indivisible and indestructible atoms.
  • All atoms of a specific element are identical in mass and properties.
  • Compounds form through combinations of different types of atoms.
  • Chemical reactions involve the rearrangement of atoms.
• Dalton's theory provided explanations for the laws of definite and multiple proportions.
• Dalton's atomic theory established a base for modern chemistry and the understanding of matter.

Discovery of Subatomic Particles

• Scientists discovered in the late 19th century that atoms can be divided into smaller subatomic particles.
• J.J. Thomson discovered the electron in 1897 while working with cathode rays.
• Thomson's "plum pudding" model pictured electrons within a positively charged sphere.
• Ernest Rutherford, with Hans Geiger and Ernest Marsden, performed the gold foil experiment in 1909.
• The gold foil experiment involved directing alpha particles at a thin gold foil and observing how they scattered.
• Most alpha particles passed through the gold foil, while some deflected at large angles.
• The deflections indicated a small, dense, positively charged nucleus inside the atom.
• Rutherford's nuclear model proposed a positively charged nucleus surrounded by electrons.
• James Chadwick discovered the neutron in 1932, completing the understanding of the atom as composed of protons, neutrons, and electrons.

Development of Quantum Mechanics

• The emergence of quantum mechanics in the early 20th century changed the understanding of the atom.
• Niels Bohr suggested in 1913 that electrons orbit the nucleus in specific energy levels or shells.
• Bohr's model could explain the discrete emission and absorption spectra of atoms.
• Louis de Broglie proposed the wave-particle duality, stating that electrons behave as waves.
• Erwin Schrödinger's Schrödinger equation, developed in 1926, describes electron behavior in atoms.
• The Schrödinger equation is a fundamental equation in quantum mechanics.
• Werner Heisenberg's uncertainty principle states the impossibility of knowing both the position and momentum of an electron perfectly.
• In the quantum mechanical model, electrons exist in orbitals, areas of high probability for finding an electron.
• Quantum numbers describe the properties of atomic orbitals.
• The quantum mechanical model provides a more accurate understanding of the atom than previous models.

Modern Atomic Theory

• Modern atomic theory combines quantum mechanics with discoveries about subatomic particles.
• Atoms are composed of protons, neutrons, and electrons.
• Protons and neutrons are located in the nucleus, which is at the center of the atom and has a positive charge.
• Electrons exist outside the nucleus within orbitals.
• The number of protons determines the element.
• Isotopes are variants of an element with differing numbers of neutrons.
• Ions are atoms that have gained or lost electrons, resulting in a net charge.t
• Modern atomic theory explains chemical bonding, molecular structure, and chemical reactions.
• Atomic theory has applications across medicine, materials science, and nanotechnology.