Forms of matter
Change of state
Matter exists in three forms (solid, liquid, and gas). The difference in form
is mainly due to different in force that held the atoms together (bonds).
Atoms are held together by some forces. These interatomic bonding forces
that hold atoms together are cohesive forces. Interatomic bonds may be
classified as:
1- Primary bonds.
2- Secondary bonds.
These are chemical in nature.
a- Ionic bonds: these are simple chemical bonds, resulting from mutual
attraction of positive and negative charges; the classic example is
sodium chloride.
b- Covalent bonds: in many chemical compounds, two valence electrons
are shared. The hydrogen molecule is an example of this bond.
c- Metallic bonds: The third type of primary atomic interaction is the
metallic bond which results from the increased spatial extension of
valence-electron wave functions when an aggregate of metal atoms is
brought close together. This type of bonding can be understood best
by studying a metallic crystal such as pure gold. Such a crystal
consists only of gold atoms. Like all other metals, gold atoms can
easily donate electrons from their outer shell and form a "cloud" of
free electrons. The contribution of free electrons to this cloud results
in the formation of positive ions that can be neutralized by acquiring
new valence electrons from adjacent atoms.
Figure (1-1): (A) Ionic bond formation
characterized by electron transfer from
one element (positive) to another
(negative). (B) Covalent bond formation
characterized by electron sharing. (C)
Metallic bond formation characterized by
electron sharing and formation of a gas
or cloud of electrons.
In contrast with primary bonds, secondary bonds weaker bonds may be said
to be more physical than chemical, they do not share electrons. Instead,
charge variations among molecules or atomic groups induce polar forces
that attract the molecules. Since there are no primary bonds between water
and glass, it is initially difficult to understand how water drops can bond to
an automobile windshield when they freeze to ice crystals. However, the
concepts of hydrogen bonding and Van Der Waals forces (two types of
bonds that exist between water and glass) allow us to explain this adhesion
phenomenon.
Van Der Waals forces: this is due to the formation of dipole. In the
symmetric atoms (e.g. inert gas) a fluctuating dipole is formed, i.e. within
an atom there is accumulation of electrons in one half leading to a negative
polarity and on the other half a positive polarity. This attracts other similar
dipoles. A permanent dipole is formed within asymmetrical molecules, e.g.
water molecule.
Figure (1-2): Hydrogen bond formation between water molecules. The polar
water molecule ties up adjacent water molecule via an H…O interaction
between molecules.