Quantum Theory and the Electronic Structure of Atoms

• Visible light is one type of electromagnetic spectrum (electromagnetic energy or radiant energy).
• 1. Frequency (ν) is the number of cycles the wave undergoes per second and is expressed in units 1/seconds (s-1 or hertz-Hz
• . Wavelength (λ) is the distance between any point on a wave and the corresponding point on the next crest (or trough) of the wave, that is, the distance the wave travels during one cycle.
• Since the product of ν and λ is a constant, then the individual terms have a reciprocal relationship to each other: radiation with high frequencies has a short wavelength, and vice versa.
• Another characteristic of a wave is its amplitude, the height of the crest (or the depth of the trough) of each wave.

  The amplitude of an electromagnetic wave is a measure of the strength of its electric and magnetic fields. Thus, the amplitude is related to the intensity of the radiation.
• When light strikes the edge of an object, it bends around it in a phenomenon called diffraction.
• When light passes a new medium (ex: water) at an angle other than 90o, the speed of light changes, causing the phenomenon known as refraction.
• Blackbody Radiation- refers to the energy radiated by any object or system that absorbs all incident radiation.
• In 1900, the German physicist Max Planck made a radical assumption that eventually led to an entirely new view of energy from blackbody radiation.

  He proposed that the hot glowing object (blackbody) could emit (or absorb) only certain quantities of energy:
• •Planck’s proposal led to the idea that if an atom can emit fixed quantities, it follows that the atom itself can have only certain quantities of energy.
• 1.Photoelectric Effect- the flow of current when monochromatic light of sufficient energy shines on a metal plate.
• •Carrying Planck’s idea of quantized energy, Albert Einstein proposed that light itself is particulate, that is, quantized into small “bundles” of electromagnetic energy, which were later called “photons”.
• Planck’s quantum theory and Einstein’s photon theory agree that energy is contained in fixed quantity and discrete particles.
• The photon model does not replace the wave model, but both are needed to understand and is now known as the dual nature of light.
• •Einstein proposed the momentum of a photon.• Louise de Broglie used the idea of Einstein to derive the wavelength of matter.
• •G. P. Thomson showed the wave-like behavior of electrons after obtaining the phenomenon: the diffraction.•J. J. Thomson proposed his model “plum pudding” with the assumption that electrons are “particles” with charge.•G. P. Thomson, his son, proved the wave-like property of electrons through diffusion.•They both won a Nobel Prize.
• Danish physicist, Niels Bohr suggested his model for the Hydrogen atom in Rutherford’s laboratory, using the ideas of Planck and Einstein. He proposed his three postulates:
• •When the electron absorbs a photon and moves to the second orbit or higher orbit, the atom is said to be in an excited state.
• •When the single electron of H is in the first orbit, the atom is said to be in its lowest (first) energy level, called the ground state.
• •In the classical view of the world, a moving particle has a definite location at any instant, whereas a wave is spread out in space.
• •But since electrons have the properties of both a particle and a wave, it is impossible to simultaneously know the exact position and momentum (mass times velocity) of a particle. This is known as Heisenberg’s Uncertainty Principle, introduced by the German physicist Werner Heisenberg in 1927.
• •The acceptance of the dual nature of matter and energy and of the uncertainty principle culminated in the field of Quantum Mechanics, which examines the wave nature of objects on the atomic scale.
• •In 1926, Erwin Schrodinger derived an equation that is the basis for the quantum-mechanical model of the hydrogen atom.
• ψ(Greek psi) is called a wave function, a mathematical description of the electron’s matter-wave in terms of position in three dimensions
• • The Schrodinger’s Equation is quite complex but is represented as: ​​  Ĥψ= E ψ
• •  Ĥ is called the Hamiltonian operator
• • In Schrodinger’s model, an atom contains an atomic orbital, but never the same in Bohr’s Model.
• Probability contour refers to the 90% volume in which electrons spend most of their time.
• 4. Electron Spin Quantum Number (ms)- represents the possible orientation of an electron in relation to another electron occupying the same orbital.
• Magnetic Quantum Number (ml)- represents the orientation of orbitals in 3D space
• Azimuthal/ Subsidiary/ Angular Momentum Quantum Number (l)- represents the energy sublevel or the type of orbital
• Principal Quantum Number (n)- represents the main energy level
• Mendeleev proposed the existence of an unknown element that he called eka- aluminum and predicted a number of its properties.
• In 1864 the English chemist John Newlands noticed that when elements were arranged in order of atomic mass, every eight element had similar properties.