Atomic Theory Development

Overview

This lecture covers the historical development of atomic theory, highlighting key scientists and experiments that shaped our current understanding of atomic structure.

Early Atomic Theory

• Democrus (500 BC) proposed that matter is made of indivisible particles called atoms, separated by empty space.
• His ideas remained largely unchanged for over 2,000 years.

Dalton's Atomic Model

• In the early 1800s, John Dalton described atoms as solid spheres.
• Dalton suggested different elements are made of different types of atoms.

Thomson's Plum Pudding Model

• In 1897, J.J. Thomson discovered atoms contain negatively charged particles (electrons).
• Thomson proposed the plum pudding model: a ball of positive charge with electrons embedded within.

Rutherford’s Nuclear Model

• In 1909, Rutherford’s gold foil experiment showed some alpha particles were deflected, disproving the plum pudding model.
• Rutherford proposed atoms have a compact, positively charged nucleus with electrons in a surrounding cloud.
• His model could not explain why electrons did not collapse into the nucleus.

Bohr’s Model and Further Developments

• In 1913, Niels Bohr suggested electrons orbit the nucleus in shells.
• Bohr’s model solved the collapse problem and forms the basis of the modern atomic model.
• Later, Rutherford discovered the nucleus contains protons (positively charged).
• In 1932, James Chadwick found evidence for neutrons (neutral particles) in the nucleus.

Key Terms & Definitions

• Atom — the smallest indivisible unit of matter.
• Electron — a negatively charged subatomic particle.
• Nucleus — the central core of the atom containing protons and neutrons.
• Proton — a positively charged particle in the nucleus.
• Neutron — a neutrally charged particle in the nucleus.
• Plum Pudding Model — Thomson’s model of an atom as a sphere of positive charge with electrons inside.
• Nuclear Model — Rutherford's model featuring a dense, positively charged nucleus.

Action Items / Next Steps

• Review the differences between each atomic model.
• Study questions and practice problems on atomic structure.
• Review related videos or readings as directed by your instructor.