Chemical bonds are forces that hold atoms together in molecules. In 1916, two types of bonds were described: ionic bonds and covalent bonds. Ionic bonds result from the transfer of electrons between atoms to achieve stable electron configurations, like the formation of lithium fluoride through the transfer of an electron from lithium to fluorine. Covalent bonds result from the sharing of electron pairs between atoms, like the sharing of an electron pair between two hydrogen atoms to form hydrogen gas. The introduction of quantum mechanics in 1926 dramatically changed the understanding of how molecules are formed through chemical bonds.

1. The chemical bond
before 1926

2. A chemical bond is a lasting attraction
between atoms, ions or molecules that enables the
formation of chemical compounds
Any consideration of the structure of molecules must
begin with a discussion of chemical bonds, the forces
that hold atoms together in a molecule

3. In 1916 two kinds of chemical bond were described:
the ionic bond by Walther Kossel (in Germany) and
the covalent bond by G. N. Lewis (of the University
of California).

4. Both Kossel and Lewis based their ideason the
following concept of the atom.
A positively charged nucleus is surrounded by
electrons arranged in concentric shells or energy
levels. There is a maximum number of electrons that
can be accommodated in each shell: two in the first
shell, eight in the second shell, eight or eighteen in
the third shell, and so on. The greatest stability is
reached when the outer shell is full, as in the noble
gases. Both ionic and covalent bonds arise from the
tendency of atoms to attain this stable configuration
of electrons.

5. The ionic bond results from transfer of electrons, as, for
example, in the formation of lithium fluoride
A lithium atom has two electrons in its inner shell and one
electron in its outer or valence shell; the loss of one electron
would leave lithium with a full outer shell of two electrons.
A fluorine atom has two electrons in its inner shell and seven
electrons in its valence shell; the gain of one electron would
give fluorine a full outer shell of eight.

6. Lithium fluoride is formed by the transfer of one electron
from lithium to fluorine; lithium now bears a positive charge
and fluorine bears a negative charge. The electrostatic
attraction between the oppositely charged ions is called an
ionic bond. Such ionic bonds are typical of the salts formed by
combination of the metallic elements (electropositive
elements) on the far left side of the Periodic Table with the
non-metallic elements (electro¬negative elements) on the far
right side.

7. The covalent bond results from sharing of
electrons, as, for example, in the formation of the
hydrogen molecule. Each hydrogen atom has a
single electron; by sharing a pair of electrons, both
hydrogens can complete their shells of two.

8. Two fluorine atoms, each with seven electrons in the
valence shell, can complete their octets by sharing a pair
of electrons. In a similar way we can visualize the
formation of HF, H20, NH3, CH4, and CF4. Here, too,
the bonding force is electrostatic attraction: this time
between each electron and both nuclei.

9. The covalent bond is typical of the compounds of
carbon; it is the bond of chief importance in the study
of organic chemistry

10. The introduction of quantum mechanics in 1926
caused a tremendous change in ideas about how
molecules are formed.

11. References
Robert T. Morrison and Robert
N. Boyd,Organic Chemistry, Sixth
Edition,PageNo.3-5

12. THANK YOU