The Atomic Model

atomicToday, many scientists simply dismiss the  classical view of the atom as little more than a footnote in history, superseded by the  development within quantum mechanics; even though the basic particle model is still part of the foundation science taught in most schools. However, just ignoring history can lead to a lack of perspective concerning the overall development of the particle model and the sequence of assumptions that still often underpin our visualisation of the sub-atomic world. It has been said, and not without some justification, that few, if any, have ever fully understood the theory behind quantum physics.  Maybe, for this reason, the particle model persists because its satisfies the preference for some form of visual model of the atomic structure, irrespective of its actual physical reality. So while the reality of quantum universe, based on wave probability may be the accepted view, it is not the starting point for this discussion. At this point, we shall continue with the more classical model of the atom as it may yet provide some insights to the general workings of an atom, which may still useful when atoms are combined into molecular and organic compounds. Our previous historical introduction left the story of the development of the atomic model in 1932, when James Chadwick discovered the neutron. However, this was really just the beginning of a model growing in complexity, which by 1968, had introduced the idea of the proton and neutron also having sub-structure. The charm quark was proposed in 1974, followed by the bottom quark in 1977 and finally the top quark in 1995. Over the years, an ever-growing number of exotic particles have been ‘discovered’ and the following list is simply an illustration, albeit a possibly bewildering one, of the complexity of the current particle model:

  • Fermions have ½, odd spin, e.g. protons, neutrons, electrons.
  • Fermions are closely associated with particles of matter.
  • Bosons have integral spin, e.g. photons.
  • Bosons are closely associated with particles of force.
  • Fermions obey the Pauli Exclusion Principle, bosons do not
  • Electrons have small mass and negative charge=0.511MeV
  • The nucleus comprises of protons and neutrons.
  • Hadrons are heavy particles made of 3 quarks, e.g. protons & neutrons
  • Quarks come in several difference variants
  • Up quarks (charge = +2/3) Down quarks (charge = -1/3)
  • Protons are ~1830x larger than electron and have positive charge
  • Proton charge = 2U+1D quark = 2/3 + 2/3 - 1/3 = 1
  • Neutrons are similar in size to proton, but have no charge
  • Neutron charge = 1U+2D quark = 2/3 - 1/3 - 1/3 = 0
  • Baryons are hadrons with the property of fermions
  • Leptons are light fermions, e.g. electrons, neutrinos
  • Neutrinos have no charge and only interact weakly with matter. 

However, in contrast to all the apparent complexity of the terminology above, most of the basic workings of the universe can still be outlined using only 3 kinds of particles:

  • Protons
  • Neutrons
  • Electrons 

These particles, combined with four fundamental forces, appear to be adequate to describe the basic workings of universe, at least, at the human and cosmic levels, if not the quantum level.

  • Gravity acts on planetary masses over huge distances
  • Electromagnetism binds atoms and molecules
  • Weak Nuclear associated with radioactive decay 
  • Strong Nuclear binds protons and neutrons within the nucleus of an atom

However, even today, particle physicists are still actively researching the mechanisms by which these forces can be described in terms of particle interactions:

Force Particle Configuration Relative
Range Attributes
Gravity Graviton Attractive Mass-to-Mass 6 * 10-39 Infinite
Electro-Magnetic Photon Attractive: Charge [+} to [-]
Repulsive: Charge [-} to [-]
Repulsive: Charge [+} to [+]
1/137 Infinite
Strong Nuclear Gluon Attractive: binds nucleus 1 10-15 Mass=?
Weak Nuclear Boson Neutrino interaction 10-6 10-18 Mass>80Gev

There is one further introductory aspect associated with the weak nuclear force called beta decay. This process describes how a neutron can decay into a proton, electron and anti-neutrino. While this process will occur for a free neutron in approximately 10 minutes, the neutron will remain stable, whilst held in the configuration of an atomic nucleus. If a neutron is made up of 1[Up] and 2 [Down] quarks, the formation of a proton requires one of the [Down] quarks to transition into an [Up] quark in order to create the positively charged proton offset by a negatively charged electron in order to comply with the conservation of charge. However, intuitively this seems to be indicative of further sub-structure mechanisms that are not really explained or well understood within the classical context of a particle. However, we shall start our examination of this complexity by first considering the assumptions that went into the development of the Bohr model of the hydrogen atom, introduced by Niels Bohr in 1913.