Class 12 Physics Modern Physics lect-03

Rutherford's Model (1911) Based on his gold foil experiment—where positively charged alpha particles were deflected by a thin sheet of gold—Rutherford proposed the Nuclear Model of the Atom. Dense Positive Nucleus: Most of an atom's mass and all of its positive charge are concentrated in a extremely small central core called the nucleus. Empty Space: The majority of the atom's volume is empty space. Planetary Electrons: Negatively charged electrons revolve around the dense nucleus in circular paths, much like planets orbiting the Sun. Key Limitation Classical physics dictates that any accelerating charged particle constantly radiates energy. If electrons were simply orbiting the nucleus, they would lose energy, spiral inward, and collapse into the nucleus within a fraction of a second. This model could not explain atomic stability or line spectra. Bohr's Model (1913) Niels Bohr modified Rutherford's model by incorporating Max Planck's quantum theory to solve the stability problem. Bohr's Quantized Orbit Model. Electrons revolve around the nucleus only in specific, fixed circular paths called energy levels or shells (labeled K,L,M,N or n=1,2,3,4). No Radiation in Stable Orbits: As long as an electron remains in a designated energy level, it does not radiate or lose energy. Energy Transitions: Electrons absorb discrete energy quanta (photons) to jump to a higher energy level (excited state) and emit energy when dropping to a lower level (ground state): ΔE=E 2 ​ −E 1 ​ =hν Quantized Angular Momentum: An electron can only orbit where its orbital angular momentum (L) is an integral multiple of 2π h ​ : mvr= 2π/nh ​