Advanced Higher Physics Introduction to Quantum Mechanics History Phenomena observed in early 20th century did not follow classical physical laws New theories were developed to account for these phenomena Starting
point taken as atomic structure Atomic Models 1 - Ancients Atom is derived from Greek ~ a meaning not (like prefix un-) tom meaning cut Greek philosophers thought that atoms were the smallest possible things, and therefore indivisible unable to be cut This
theory was widely accepted to be true until the late 19th century Atomic Models 2 - Thomson 1897 Thomsons discovery of electron leads to Plum Pudding model Large positive mass with randomly
arranged negative charges Atomic Models 3 - Rutherford 1909 Scattering experime nt not consistent with Thomson model Rutherford postulated nucleus containing positive charges with
electrons in orbits like planets Atomic Models 3 (cont.) Later work lead to the discovery of The proton (Rutherford -1919) The neutron
(Chadwick 1932) Atomic Models 4 - Bohr Rutherford model still unable to explain spectral lines associated with emission of light from atoms Atomic Models 4 (cont.) Bohrs model has electrons in orbit around a central nucleus, but allows only certain orbits for electrons. For stable orbit, angular momentum must
Angular be a multiple of h / 2 momentum of electrons is nh mvr quantised 2 n, is order of electron level Atomic Models 4 (cont.)
Reasons for electron stability related to De Broglie wavelength n=6 Treating the orbit of an electron as a continuous wave, the path length (2r) must be equal to a whole number of wavelengths i.e. n = 2r Graphical Representation Atomic Models 4 (cont.) From
De Broglie equation, Combining with, n 2r nh 2r mv nh mvr 2 nh
mvr 2 h mv Atomic Models 4 (cont.) Angular momentum of electron in any orbit is always a multiple of h / 2 This quantum of angular momentum is
often expressed as (h bar), where = h / 2 Scholar Bohr Hydrogen Atom demo Energy Levels For any quantum number, n, there exists a single orbit with a specific angular momentum, L = mvr, and energy, E, which can be calculated.
Each quantum number, n, relates to an electron energy level, En, in the atom. When electrons move between energy levels they either absorb energy (excite) or emit energy (de-excite) Spectral Lines 1 When an electron gains energy, by absorbing a
photon, it rises to a higher energy level (excitation) When an electron loses energy, by emitting a photon, it falls to a lower energy level (de-excitation) Spectral Lines 2 Hydrogen has a number of groups or series of line spectra, each for transitions to the same lower energy level.
Scholar Hydrogen emission demo