Atomic Models: Rutherford to Quantum Mechanics
Rutherford Model
The atom has a small, central nucleus with a positive electric charge, containing nearly all of the atom’s mass. Electrons with negative electric charge orbit the nucleus in long-distance circular paths. The electrical attraction between electrons and the nucleus provides the centripetal force for this revolution. The sum of the negative charges of the electrons equals the positive charge of the nucleus, making the atom electrically neutral.
Bohr Atomic Model
Postulate 1: Electrons orbit the nucleus in stationary orbits without emitting energy.
Postulate 2: Orbits are only possible where the angular momentum of the electron is a multiple of h/2π.
Postulate 3: When an electron moves from a higher orbit to a lower orbit, the energy difference is emitted as electromagnetic radiation.
Planck Theory
The emission of electromagnetic radiation occurs in discrete amounts called elementary quanta.
Wave-Particle Duality (de Broglie Principle)
Masses in motion can behave as waves. This duality is a general property of matter, not just photons.
Heisenberg Uncertainty Principle
It is impossible to measure simultaneously and with absolute precision the value of two conjugate variables (e.g., energy and time, or position and momentum).
Heisenberg and Schrödinger Comparison
Heisenberg’s matrix mechanics represents magnitudes like speed, force, or position with matrices. Schrödinger’s wave mechanics is a classical wave theory of matter. Max Born interpreted it using statistical methods, showing it differs from classical mechanics.
Schrödinger Model
The electron is described as a wave vibrating around the nucleus, known as wave mechanics. The Schrödinger equation defines the orbital, a region of space where there is a high probability of finding the electron.
Pauli Exclusion Principle
No two electrons in a single atom can have the same four quantum numbers.
Hund’s Rule of Maximum Multiplicity
Electrons are placed singly in each sublevel orbital whenever possible, avoiding pairing in the same orbital.
Atomic Radius
Group: Atomic radii increase with increasing atomic number because electrons are placed in higher energy levels, further from the nucleus. The repulsion from inner electrons also increases the volume.
Period: Atomic radii decrease with increasing atomic number. Each element has one more electron, but these are located in the same or lower energy levels, so the atom does not increase in size.