Important Terms and Definitions in Chemical Bonding
Chemical Bond
** Molecules of
chemical substances are made of two or more atoms joined together by some force,
acting between them. This force which results from the interaction between the
various atoms that go to form a stable molecule, is referred to as a Chemical Bond.
** A chemical
bond is defined as a force that acts between two or more atoms to hold them together
as a stable molecule.
** There are three different types of bonds recognised by
chemists :
(1) Ionic or
Electrovalent bond
(2) Covalent
bond
(3) Coordinate
covalent bond
There is a
fourth type of bond, namely, the metallic bond which we will consider later in
this chapter.
Definition of Valence
** The term
valence (or valency) is often used to state the potential or capacity of an
element to combine with other elements.
** At one time,
it was useful to define valence of an element as : the number of hydrogen atoms
or twice the number of oxygen atoms with which that element could combine in a
binary compound (containing two different elements only).
** In hydrogen
chloride (HCl), one atom of chlorine is combined with one atom of hydrogen and
the valence of chlorine is 1.
** In magnesium
oxide (MgO), since one atom of magnesium holds one atom of oxygen, the valence
of magnesium is 2.
** By the above
definition, we would assign a valence of 2 to sulphur in H2S, but 4
to sulphur in SO2. Some elements have fractional valence in certain
compounds, while there are elements that have variable valencies.
** The concept
of valence as a mere number could not explain these facts. This concept, in
fact, was very confusing and has lost all value.
** As already
stated, there are three different type of bonds that are known to join atoms in
molecules. Although no precise definition of valence is possible, we can say
that : Valence is the number of bonds formed by an atom in a molecule.
** Valence
is the number of bonds formed by an atom in a molecule.
Valence Electrons
** The
electrons in the outer energy level of an atom are the ones that can take part
in chemical
bonding. These
electrons are, therefore, referred to as the valence electrons.
** The
electronic configuration of Na is 2, 8, 1 and that of Cl is 2, 8, 7. Thus
sodium has one valence electron and chlorine 7.
** It is
important to remember that for an A group element of the periodic table (H, O,
K, F, Al etc.) the group number is equal to the number of valence electrons.
Bonding and Non-bonding Electrons
** The valence
electrons actually involved in bond formation are called bonding electrons.
** The remaining
valence electrons still available for bond formation are referred to as
non-bonding electrons.
Lewis Symbols of Elements
** A Lewis
symbol of an element consists of an element’s symbol and surrounding dots to represent
the number of valence electrons.
** In this
notation, the symbol of an element represents the nucleus plus the inner
normally filled levels (or shells) of the atom. For illustration, the symbol Na
stands for the nucleus of sodium atom plus 2, 8 electrons in the inner two
levels.
** To represent
a Lewis symbol for an element:
(1) write down
the symbol of the element and surround the symbol with a number of dots (or
crosses) equal to the number of valence electrons.
(2) The
position of dots around the symbol is not really of any significance.
(3) The bonding
electrons are shown at appropriate positions, while the rest of the electrons
are generally given in pairs.
The Lewis
symbols for hydrogen, chlorine, oxygen and sulphur may be written as :
** The
structural formulae of compounds built by union of Lewis symbols for the
component atoms, are referred to as Electron-dot formulas, or Electron-dot structures
or Lewis structures.
** For this purpose,
the valence electrons actually involved in bond formation may be shown by crosses
(x) or dots (.) for the sake of distinction.
Electronic Theory of Valence
** As Bohr put
forward his model of the atom so electronic configuration of elements was
known. G.N. Lewis and W. Kossel, working independently, used this knowledge to
explain ‘why atoms joined to form molecules’. They visualised that noble gas
atoms had a stable electronic configuration, while atoms of all other elements
has unstable or incomplete electronic configuration. In 1916, they gave the electronic
theory of valence.
** The electronic theory of valence states that :
In chemical bond formation, atoms interact by losing, gaining, or sharing of
electrons so as to acquire a stable noble gas configuration.
** Each noble
gas, except helium, has a valence shell of eight electrons (see Table).
** While atoms
of noble gases possess a stable outer shell of eight electrons or octet, atoms
of most other elements have incomplete octets. They may have less than 8
electrons or in excess. Thus, the electronic theory or valence could well be
named as the Octet theory of Valence.
** The Octet
theory of Valence may be stated as : Atoms interact by electron-transfer or
electron-sharing, so as to achieve the stable outer shell of eight electrons.
** The tendency
for atoms to have eight electrons in the outer shell is also known as the Octet
Rule or the Rule of Eight. Since helium has two electrons in the outer shell,
for hydrogen and lithium, having one and three (2, 1) electrons respectively,
it is the Rule of two which will apply. There are quite a few exceptions to the
rule of eight in covalent compounds.
Reference: Essentials of Physical Chemistry /Arun Bahl, B.S Bahl and G.D. Tuli / multicolour edition.
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