molecules, atoms are held together by attractive forces, called chemical bonds.
Kossel and Lewis are the first people to provide a logical explanation for
chemical bonding. They proposed that atoms try to attain the nearest noble gas
electronic configuration by losing, gaining or sharing one or more electrons
during the bond formation. The noble gases contain eight electrons in their
valance shell which is considered to be stable electronic configuration. The
idea of Kossel – Lewis approach to chemical bond lead to the octet rule, which
states that “the atoms transfer or share electrons so that all atoms involved
in chemical bonding obtain 8 electrons in their outer shell (valance shell)”.
are different types of chemical bonds. In compounds such as sodium chloride,
the sodium atom loses an electron which is accepted by the chlorine atom
resulting in the formation of Na+ and Cl- ions. These two
ions are held together by the electrostatic attractive forces. This type of
chemical bond is known as ionic bonds or electrovalent bonds. In certain
compounds, instead of the complete transfer of electrons, the electrons are
shared by both the bonding atoms. The two combining atoms are held together by
their mutual attraction towards the shared electrons. This type of bond is
called covalent bonding. In addition, there also another bond type known as
coordinate covalent bonds, where the shared electrons of a covalent bond are
provided by only one of the combining atoms. Metallic bonding is another type
of bonding which is observed in metals.
theory in combination with VSEPR theory will be useful in predicting the shape
of molecules. According to this theory, the shape of the molecules depends on
the number of valance shell electron pair (lone pairs and bond pairs) around
the central atom. Each pair of valance electrons around the central atom repels
each other and hence, they are located as far away as possible in
three-dimensional space to minimise the repulsion between them.
Heitler and London gave a theoretical treatment to explain the
formation of covalent bond in hydrogen molecule on the basis of wave mechanics
of electrons. It was further developed by Pauling and Slater. According to this
theory when half-filled orbitals of two atoms overlap, a covalent bond will be
formed between them. Linus Pauling introduced the concept of hybridisation.
Hybridisation is the process of mixing of atomic orbitals of the same atom with
comparable energy to form equal number of new equivalent orbitals with same
energy. There are different types of hybridization such as sp, sp2, sp3, sp3d2
Hund and Robert. S. Mulliken developed a bonding theory called molecular
orbital theory. According to this theory, when atoms combines to form
molecules, their individual atomic orbitals lose their identity and forms new
orbitals called molecular orbitals. The filling of electrons in these orbitals
follows Aufbau's principle, Pauli's exclusion principle and Hund's rule as in
the case of filling of electrons in atomic orbitals.