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Structure
and bonding - Pt 2
Francine
Taylor-Campbell, Contributor
 |
| A
student of the Inswood High School has to walk on these chairs to leave the school's
property, due to flooding from the heavy rains that have lashed the parish of
St. Catherine, earlier this year. - Ricardo Makyn/Staff Photographer |
YOU SHOULD
BE ABLE TO Explain
metallic bonding using the terms cation and mobile electrons. Define
and give examples of ionic crystals, simple molecular and giant molecular crystals.
Distinguish
between ionic and molecular solids. POINTS
TO NOTE - Metals
are able to conduct electricity and heat due to the presence of mobile electrons.
- Solids
can be divided into the following groups: ionic crystals, molecular (simple and
giant) and metallic crystals. The differences in the properties of these compounds
can be explained by the type of bonds they possess.
- Ionic
compounds are crystalline solids, able to conduct electricity when molten, due
the movement of ions which can carry an electric current. These compounds also
have high melting and boiling temperatures due to the strength of the attractive
forces between the ions.
- Simple
molecular crystals have low melting and boiling temperatures due to weak forces
of attraction between molecules.
- Giant
structures of atoms and molecules have very high melting and boiling points due
to strong covalent bonds throughout their three-dimensional network.
The
arrangement of atoms and ions in a crystal helps to determine the physical properties
of thermal and electrical conductivity, melting and boiling points, physical state
at room temperature and solubility in solvents. In
metallic crystals, the outer electrons of each atom are mobile or delocalised
(that is, they do not belong to any particular cation) and come together to form
a 'band or sea' of electrons. These electrons will bind to the cations formed
from the electron loss forming a strong bond. In this way, metals are able to
conduct heat and electricity since the mobile electrons can move throughout the
metal. The strong bonds between the cations and electrons means that they are
hard to break thus, metals have high melting and boiling points. Metals are also
solids (except mercury) and are malleable and ductile. The bonding in metals can
be represented by the diagram below. º+
e º+ e º+ eº + e The
mobile electrons form a cloud or band surrounding the cations. The difference
in charges holds them together into a strong bond. In
an ionic crystal, the attraction between cations and anions holds the crystal
together into a regular three-dimensional framework. Each cation is surrounded
by anions and vice versa. These crystals are solids at room temperature and are
unable to conduct electricity in this state. However, imagine what happens when
these ionic crystals are heated; the ions gain more energy to move but because
they are oppositely charged, it requires vast amounts of energy to break this
force of attraction and so, these crystals have high melting and boiling points.
Note: Ionic solids can only conduct electricity when molten, as only then are
the ions free enough to move. Examples of ionic solids are sodium chloride,
magnesium oxide and potassium iodide. In
giant molecular crystals such as graphite, diamond and silicon dioxide, strong
covalent bonds exist between the atoms, which make them difficult to melt or boil.
On the other hand, simple molecular crystals have covalent bonds within molecules
but weak bonds between molecules. Hence, the molecules separate easily at fairly
low temperatures. Attempt
to explain the following: 1.
Most ionic crystals are solids while simple covalent molecules are not. 2.
Sodium chloride does not conduct electricity in its solid state. 3.
Metals can conduct heat and electricity. In
next week's lesson, we will examine in greater detail the differences in properties
and bonding between sodium chloride, diamond and graphite. Continue
to review these points and practice questions from your text. Francine
Taylor-Campbell is an independent contributor. | 
