2.1 Atomic Theory and Subatomic Particles

Atoms are the building blocks of matter, and our understanding of them has evolved dramatically over time. From ancient Greek philosophers to modern quantum mechanics, scientists have gradually uncovered the intricate structure of these tiny particles.

At the heart of an atom lies its nucleus, surrounded by an electron cloud. Protons, neutrons, and electrons make up these fundamental components, with isotopes adding further complexity to atomic structure. This knowledge forms the foundation for countless scientific and technological advancements.

Historical Development and Structure of Atoms

Evolution of atomic theory

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• Ancient Greek philosophers introduced concept of indivisible particles (atoms) through Democritus
• John Dalton proposed all matter composed of atoms in early 19th century
• J.J. Thomson discovered electrons and developed "plum pudding" model with electrons embedded in positive mass
• Ernest Rutherford's gold foil experiment revealed dense, positively charged nucleus
• Niels Bohr proposed planetary model with electrons orbiting nucleus in fixed energy levels
• Erwin Schrödinger developed quantum mechanical model describing electrons as probability clouds

Components of atoms

• Protons carry positive charge, found in nucleus, mass 1.67 × 10^-27 kg
• Neutrons electrically neutral, located in nucleus, slightly heavier than protons
• Electrons negatively charged, orbit nucleus in electron clouds, mass 9.11 × 10^-31 kg

Structure of atoms

• Nucleus forms central core containing protons and neutrons, holds most of atom's mass
• Electron cloud surrounds nucleus, divided into energy levels or shells
• Atomic number equals number of protons, determines element's identity (Hydrogen: 1, Helium: 2)
• Mass number sums protons and neutrons in nucleus (Carbon-12: 6 protons + 6 neutrons)

Isotopes and their variations

• Atoms of same element with different neutron counts
• Share atomic number but have different mass numbers
• Chemical properties largely unchanged
• Carbon isotopes include C-12, C-13, C-14 with 6, 7, and 8 neutrons respectively
• Hydrogen isotopes: H-1 (protium), H-2 (deuterium), H-3 (tritium)
• Applications include medical imaging (Technetium-99m), cancer treatment (Iodine-131), and archaeological dating (Carbon-14)