Attraction between atoms or ions leads to a
chemical bond. According to the types of bonds contained in a molecule,
the physical properties including melting point, hardness, electrical
and thermal conductivity and solubility are determined. Chemical bonds
involve only the outermost or valence electrons of atoms. Using the
example of the simplest element, hydrogen, its two atoms on approaching
each other, cause electon-electron and proton-proton repulsions to
attempt separation of the atoms. But counterbalance by proton-electron
attraction fuses the two hydrogen atoms forming a bond.
This example demonstrates the gain, loss and
sharing of electrons by atoms for possession of the same number of
electrons as the noble gas in closest proximity on the periodic table.
With eight valence electrons (s2p6), all noble gases are chemically
stable in a phenomenon called the octet rule. But certain exceptions
are possible. Among them, one group of atoms has less than eight
electrons like hydrogen with only one. BeH2 has just four valence
electrons around Be, with Beryllium contributing two electrons and each
hydrogen with one. The second exception applies in elements in periods
4 and above. Their atoms can have over four surrounding valence pairs
in certain compounds.
Types of Chemical Bonds
Familiarity with three types of chemical bonds is
required for the SAT II Chemistry exam, ionic bonds, covalent bonds and
metallic bonds.
Ionic Bonds
An electrostatic attraction between ions with
opposite charges, cations and anions causes ionic bonds. They usually
involve metals and nonmetals as elements active in ionic bonds are
mostly from opposite ends of the periodic table with an
electronegativity difference exceeding 1.67. Being very strong, ionic
bonds in compounds increase melting points and take a solid form in
normal conditions. Finally, an electron in an ionic bond is transferred
from the less electronegative atom to the more electronegative element.
A prime example of an ionic bond-content molecule is NaCl or table
salt.
Covalent Bonds
Atoms may share electrons instead of transferring
them from atom to atom, resulting in covalent bonds. But the sharing is
rarely ever equal due to the difference in electronegativity value of
each atom. The only exception is the bond between two atoms of the same
element. Covalent bonds are said to be non-polar when the difference in
electronegativity of two atoms ranges from 0 to 0.4. Polar refers to
electronegativity difference between 0.4 and 1.67. For both non-polar
and polar covalent bonds, higher electronegativity in an element
results in stronger attraction of the electron pair. Carbon dioxide,
CO2 molecules have two bonds which are covalent bonds.
It is possible for covalent bonds to be single,
double or triple. A single bond occurs when only one pair of electrons
is shared. This single bond is a sigma bond with the electron density
being most pronounced along the line joining the two atoms.
Metallic Bonds
Metallic bonds are exclusive to metals alone,
including aluminium, gold, copper and iron. Each atom in a metal shares
a bond with a number of other metal atoms, thereby allowing their
electrons unrestricted movement within the metal structure. It is this
specific phenomenon that is behind the properties of metals being
unique, an example being their high conductivity.
That basically sums up the essence of chemical
bonds, the differences in various types and the reasons for molecular
structures being formed.
About The Author
Dr. George Grant is an experienced researcher in
Bio-chemistry. He has done extensive researches and experiments in the
field. He is a visiting faculty for some of the most reputed Science
colleges. For more information on Chemistry Tools and Definitions,
please visit: http://www.chemicool.com.
chemicool1@yahoo.com
