2.1 Nature of Matter
13
covalent bond is call ‘polar’. If one atom holds one or more extra electrons to itself
or releases one or more electrons to its neighbors, the atom has become ionized.
The coupling holding oppositely charged ions is called an ‘ionic bond’, and the
force ‘Coulombic’.
Now consider a large number of atoms each with some kinetic energy and
found in some region of space. After some time, collisions between atoms will
distribute their kinetic energies. If the resulting average kinetic energy is less
than the atomic binding energy, then molecules can form with some permanence.
Residual weak electromagnetic forces between atoms and molecules, insufficient to
cause lasting bonds at the prevailing temperature, are called ‘van der Waals forces’. 5
Van der Waals forces include weak hydrogen bonding, permanent and induced
charge multipolar interactions, and dispersion forces, i.e. forces due to the quantum
fluctuation of charges. (See Sect. 11.4 for more on molecular forces.) Van der Waals
forces between molecules diminish dramatically as the molecules are pulled apart,
dropping at least as the inverse seventh power of the separation distance.
A material will be a gas if the weak intermolecular forces cannot keep the
material localized against thermal outward pressure. Otherwise, the material will be
a liquid or a solid. Liquids are distinguished from solids by flowing under shearing
force. 6 Layers of a fluid move continuously in response to a shear. Solids can sustain
a shear statically.
If, in a solid or liquid, some fraction of the valence electrons are not captured
and held by neighboring atoms, but rather form a delocalized ‘sea’ around the
positive atoms, the material is a metal. Compared to non-metals, metals are good
conductors of electricity, and, when polished, good reflectors, all because of those
loose electrons. The electrons also make metals good conductors of heat, but some
non-metals are also good thermal conductors, such as diamonds, because the atoms
within are so tightly coupled. If the valence electrons in a solid are localized at
low temperatures, but some significant fraction of them become delocalized at room
temperatures or by a weak electric field (typically 0.2 V/mm), then that solid is
called a semi-conductor. If some atoms or molecules in a material hold a positive
or negative charge (but the net charge within volumes larger than the atoms or
molecules approaches zero as the volume increases), then the material is called
ionic. If the material is in a gas phase with delocalized electrons as a sea around
positive ions, the material is called a plasma. The solar wind is a plasma.
5 Johannes Diderik van der Waals, Ph.D. Thesis (1873).
6 A shearing force will exist if a segment of a material initially in the shape of a cube has a tangential
force on one of the surfaces of the cube, with an oppositely directed force on the opposite side of the
cube. You can shear your textbook by putting one of your hands on the top of a book with fingers
toward the binding, the other hand on the bottom cover and then moving your hands sideways but
in opposite direction, causing the textbook pages to slip a little, one to the next, limited by the book
binding.
13
covalent bond is call ‘polar’. If one atom holds one or more extra electrons to itself
or releases one or more electrons to its neighbors, the atom has become ionized.
The coupling holding oppositely charged ions is called an ‘ionic bond’, and the
force ‘Coulombic’.
Now consider a large number of atoms each with some kinetic energy and
found in some region of space. After some time, collisions between atoms will
distribute their kinetic energies. If the resulting average kinetic energy is less
than the atomic binding energy, then molecules can form with some permanence.
Residual weak electromagnetic forces between atoms and molecules, insufficient to
cause lasting bonds at the prevailing temperature, are called ‘van der Waals forces’. 5
Van der Waals forces include weak hydrogen bonding, permanent and induced
charge multipolar interactions, and dispersion forces, i.e. forces due to the quantum
fluctuation of charges. (See Sect. 11.4 for more on molecular forces.) Van der Waals
forces between molecules diminish dramatically as the molecules are pulled apart,
dropping at least as the inverse seventh power of the separation distance.
A material will be a gas if the weak intermolecular forces cannot keep the
material localized against thermal outward pressure. Otherwise, the material will be
a liquid or a solid. Liquids are distinguished from solids by flowing under shearing
force. 6 Layers of a fluid move continuously in response to a shear. Solids can sustain
a shear statically.
If, in a solid or liquid, some fraction of the valence electrons are not captured
and held by neighboring atoms, but rather form a delocalized ‘sea’ around the
positive atoms, the material is a metal. Compared to non-metals, metals are good
conductors of electricity, and, when polished, good reflectors, all because of those
loose electrons. The electrons also make metals good conductors of heat, but some
non-metals are also good thermal conductors, such as diamonds, because the atoms
within are so tightly coupled. If the valence electrons in a solid are localized at
low temperatures, but some significant fraction of them become delocalized at room
temperatures or by a weak electric field (typically 0.2 V/mm), then that solid is
called a semi-conductor. If some atoms or molecules in a material hold a positive
or negative charge (but the net charge within volumes larger than the atoms or
molecules approaches zero as the volume increases), then the material is called
ionic. If the material is in a gas phase with delocalized electrons as a sea around
positive ions, the material is called a plasma. The solar wind is a plasma.
5 Johannes Diderik van der Waals, Ph.D. Thesis (1873).
6 A shearing force will exist if a segment of a material initially in the shape of a cube has a tangential
force on one of the surfaces of the cube, with an oppositely directed force on the opposite side of the
cube. You can shear your textbook by putting one of your hands on the top of a book with fingers
toward the binding, the other hand on the bottom cover and then moving your hands sideways but
in opposite direction, causing the textbook pages to slip a little, one to the next, limited by the book
binding.