Key Concepts: Atomic Theory

Core Definitions

• Democritus: Greek philosopher who first proposed that matter is made of small, indivisible particles called 'atomos.'
• Dalton's Atomic Theory: All matter is made of atoms; atoms of the same element are identical; atoms combine in whole-number ratios.
• Thomson's Model: Discovered the electron; proposed the 'plum pudding' model with negative charges in positive mass.
• Rutherford's Model: Discovered the nucleus through the gold foil experiment; most of the atom is empty space.
• Bohr's Model: Electrons orbit the nucleus in fixed energy levels (like planets around the sun).
• Modern Model: The quantum mechanical model; electrons exist in probability clouds (orbitals), not fixed paths.