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Evolution of Atomic Models

Early Concepts of Atoms

• In the 5th century BC, Leucippus proposed that matter could be divided into smaller pieces until reaching an indivisible point.

• Democritus named this smallest piece "atom," meaning "uncuttable" or "indivisible."

• John Dalton's atomic model in 1808 revived these ideas, describing atoms as solid spheres.

Advancements in Atomic Theory

• In 1897, J.J. Thomson introduced the idea of a positively charged sphere with negatively charged electrons embedded within it, known as the "plum pudding model."

• Ernest Rutherford's experiments in 1911 revealed that atoms are not solid but have a central nucleus containing positive charge, with electrons orbiting around it.

Structure of the Atom

• Niels Bohr (1913) identified two types of particles in the nucleus: protons (positive charge) and neutrons (no charge), with electrons orbiting at defined energy levels.

• He termed the outermost electrons as "valence electrons," responsible for bonding and defining an atom's characteristics.

Further Developments

• In 1916, Sommerfeld argued that electron orbits are elliptical rather than circular and introduced sub-levels within energy levels.

• Erwin Schrödinger proposed a mathematical model in 1924 called the wave mechanical model, emphasizing probabilities over fixed locations for electrons.

Modern Understanding of Electrons