14
2 The Kinds of Ordinary Materials
Ordinary materials, whether gas, liquid, or solid, contain interacting atoms or
molecules. A common property of ordinary material particles is the repulsive nature
of the force between them when they are brought in proximity. For atoms in such
materials, this repulsion is due to the behavior of the valence electrons in atoms,
which experience mutual electrical repulsion, and a quantum mechanical repulsion
between electrons.
Quantum theory predicts that no two fermions may occupy the same quantum
state at the same time. All particles which have a spin quantum number of
1/2, 3/2, 5/2, . . . are fermions. Those with integer spin quantum number are
called bosons. Electrons and protons are fermions. If two electrons are brought
together and are forced to stay nearby over a period of time, with their spins
aligned, they will be close to a single quantum state. Quantum theory predicts that
this configuration becomes more and more unlikely as the electrons approach one
another. This property was first proposed by Wolfgang Pauli in 1925, to account for
atomic structure, before the formal quantum argument was realized. The behavior
is now referred to as the ‘Pauli exclusion principle’. The available quantum states
for electrons in the lowest energy states within atoms and molecules are few. For
atoms with atomic number greater than hydrogen, the lowest states are filled with
electrons, and cannot accept any more. Any added electrons must go into quantum
states further from the atomic nucleus, and have higher energy than the inner filled
states. Without the exclusion effect, the electrons would all fall to the lowest energy
state, and atoms would collapse, obliterating all of chemistry and biology!
Collision forces act on gas molecules to oppose compression. These momentary
forces come from the same atomic repulsion experienced constantly by molecules
in solids and liquids and also account for the repulsive force from a container
wall, opposing gas expansion and liquid encroachment. If the material is confined,
there are also forces present to keep the constituents together. In the case of
solids, bonding between atoms and molecules is due to an attraction coming from
electrical effects and from quantum properties. Atoms and molecules in liquids
feel a longer-range but weak attractive force, combining electrical and quantumfluctuation effects. As described in Sect. 11.7.7, the quantum-fluctuation forces, or
dispersion forces, come about from the induced-electric-dipolar attraction caused
by quantum jiggling of the electronic states in atoms and molecules. In general,
magnetic forces have a much less significant role in the binding of ordinary nonferrous materials. 7
A system will be self confined and in mechanical equilibrium if internal forces
oppose attempts to expand or compress that system. Adding energy to a system
7 The work of P.A.M. Dirac in 1931 implies that if particles with only north or only south magnetic
poles existed, the strength of the smallest magnetic charge g would be related to the electric charge
e by g = ¯
hc/(2e), so that north- and south- pole particles would have attracted with a force 4692
times greater than electrons and protons. Their primordial binding would explain why we observe
the remaining magnetic effects in nature as relatively weak.
2 The Kinds of Ordinary Materials
Ordinary materials, whether gas, liquid, or solid, contain interacting atoms or
molecules. A common property of ordinary material particles is the repulsive nature
of the force between them when they are brought in proximity. For atoms in such
materials, this repulsion is due to the behavior of the valence electrons in atoms,
which experience mutual electrical repulsion, and a quantum mechanical repulsion
between electrons.
Quantum theory predicts that no two fermions may occupy the same quantum
state at the same time. All particles which have a spin quantum number of
1/2, 3/2, 5/2, . . . are fermions. Those with integer spin quantum number are
called bosons. Electrons and protons are fermions. If two electrons are brought
together and are forced to stay nearby over a period of time, with their spins
aligned, they will be close to a single quantum state. Quantum theory predicts that
this configuration becomes more and more unlikely as the electrons approach one
another. This property was first proposed by Wolfgang Pauli in 1925, to account for
atomic structure, before the formal quantum argument was realized. The behavior
is now referred to as the ‘Pauli exclusion principle’. The available quantum states
for electrons in the lowest energy states within atoms and molecules are few. For
atoms with atomic number greater than hydrogen, the lowest states are filled with
electrons, and cannot accept any more. Any added electrons must go into quantum
states further from the atomic nucleus, and have higher energy than the inner filled
states. Without the exclusion effect, the electrons would all fall to the lowest energy
state, and atoms would collapse, obliterating all of chemistry and biology!
Collision forces act on gas molecules to oppose compression. These momentary
forces come from the same atomic repulsion experienced constantly by molecules
in solids and liquids and also account for the repulsive force from a container
wall, opposing gas expansion and liquid encroachment. If the material is confined,
there are also forces present to keep the constituents together. In the case of
solids, bonding between atoms and molecules is due to an attraction coming from
electrical effects and from quantum properties. Atoms and molecules in liquids
feel a longer-range but weak attractive force, combining electrical and quantumfluctuation effects. As described in Sect. 11.7.7, the quantum-fluctuation forces, or
dispersion forces, come about from the induced-electric-dipolar attraction caused
by quantum jiggling of the electronic states in atoms and molecules. In general,
magnetic forces have a much less significant role in the binding of ordinary nonferrous materials. 7
A system will be self confined and in mechanical equilibrium if internal forces
oppose attempts to expand or compress that system. Adding energy to a system
7 The work of P.A.M. Dirac in 1931 implies that if particles with only north or only south magnetic
poles existed, the strength of the smallest magnetic charge g would be related to the electric charge
e by g = ¯
hc/(2e), so that north- and south- pole particles would have attracted with a force 4692
times greater than electrons and protons. Their primordial binding would explain why we observe
the remaining magnetic effects in nature as relatively weak.