Solid State Physics
259
helps to bind the positive ions. That this leads to a lower energy state can be
seen from the following arguments.
An electron in an isolated atom is confined to a small volume around the
nucleus. This confinement gives rise to an uncertainty in the momentum,
∆p ~ /r, where r is the radius of the atom. Consequently, the electron has a
fairly substantial amount of kinetic energy, of the order of several eV. However,
in the crystalline, metallic state, the electrons are essentially free to be anywhere
in the entire crystal. As a result, there is a considerable reduction in their kinetic
energy. This is the source of metallic bonding.
The bond between two metallic atoms is somewhat weaker than ionic or
covalent bonds. This leads to relatively low melting points, for example 63°C
for K. However, the cohesive energy of metals is fairly large since each valence
electron interacts with several ions. The metallic bonds are not directional which
allows the planes of atoms to slide over each other quite easily. Hence, metals
are found to be ductile and malleable rather than brittle. The existence of
essentially free electrons gives rise to high electrical and thermal conductivity
for metals.
Van der Waals Bonds
When two neutral atoms approach each other, there is an attractive force between
them because of the induced electric dipole moments (see Sec. 5.6). These forces,
through weak, are important for atoms which do not form ionic, covalent or
metallic bonds with each other. They lead to the solidification of inert gases He,
Ne, Ar, Kr, Xe, and some organic molecules, such as methane. The binding due
to van der Waals forces is usually weak which results in a low melting point,
e.g. melting point of Ar is – 189°C, for these solids. They are also soft, electrically
insulating and generally insoluble.
Hydrogen Bonds
The hydrogen atom has only one electron and would normally form a covalent
bond with only one other atom. However, if the other atom is strongly
electronegative, the electron may be transferred to the electronegative atom.
The remaining proton being small in size, about 10
–15
m compared to the usual
atomic size of 10
–10
m, can bind only two neighbouring negative ions (the two
spheres would be glued together by the proton in-between). This gives rise to
what is known as the hydrogen bond which connects only two atoms.
The hydrogen bond is important in the formation of ice and in the
polymerization of hydrogen fluoride.
259
helps to bind the positive ions. That this leads to a lower energy state can be
seen from the following arguments.
An electron in an isolated atom is confined to a small volume around the
nucleus. This confinement gives rise to an uncertainty in the momentum,
∆p ~ /r, where r is the radius of the atom. Consequently, the electron has a
fairly substantial amount of kinetic energy, of the order of several eV. However,
in the crystalline, metallic state, the electrons are essentially free to be anywhere
in the entire crystal. As a result, there is a considerable reduction in their kinetic
energy. This is the source of metallic bonding.
The bond between two metallic atoms is somewhat weaker than ionic or
covalent bonds. This leads to relatively low melting points, for example 63°C
for K. However, the cohesive energy of metals is fairly large since each valence
electron interacts with several ions. The metallic bonds are not directional which
allows the planes of atoms to slide over each other quite easily. Hence, metals
are found to be ductile and malleable rather than brittle. The existence of
essentially free electrons gives rise to high electrical and thermal conductivity
for metals.
Van der Waals Bonds
When two neutral atoms approach each other, there is an attractive force between
them because of the induced electric dipole moments (see Sec. 5.6). These forces,
through weak, are important for atoms which do not form ionic, covalent or
metallic bonds with each other. They lead to the solidification of inert gases He,
Ne, Ar, Kr, Xe, and some organic molecules, such as methane. The binding due
to van der Waals forces is usually weak which results in a low melting point,
e.g. melting point of Ar is – 189°C, for these solids. They are also soft, electrically
insulating and generally insoluble.
Hydrogen Bonds
The hydrogen atom has only one electron and would normally form a covalent
bond with only one other atom. However, if the other atom is strongly
electronegative, the electron may be transferred to the electronegative atom.
The remaining proton being small in size, about 10
–15
m compared to the usual
atomic size of 10
–10
m, can bind only two neighbouring negative ions (the two
spheres would be glued together by the proton in-between). This gives rise to
what is known as the hydrogen bond which connects only two atoms.
The hydrogen bond is important in the formation of ice and in the
polymerization of hydrogen fluoride.