It is a well known fact that matter is made up of atoms but the atoms (other than noble gases) do not exist as independent entities under normal conditions. However, the smallest particles of matter which exist as independent entities are molecules. The molecules are nothing but the group or cluster of atoms of same or different elements which behave as units. For example, in hydrogen molecule two H atoms whereas in oxygen molecule two O atoms are held together. Water molecule contains two H atom and one 0 atom mutually held together by some attractive forces. In addition to this, there is another category of substances in which constituent units are ions of opposite charges instead of atoms or molecules. Sodium chloride is a common example of this type which contains sodium (Na+) and chloride ions (CI-) held together. The attractive force which holds various constituents atoms, ions, etc.) together in differentc hemical species is called chemical bond. The chemical bonds can be classified into various types like ionic bond, covalent bond, co-ordinate bond, metallic bond, hydrogen bond and van der Waal forces. Since the formation of chemical compounds takes place as result of combination of atoms of various elements in different , it raises many questions such as:
• Why do atoms combine?
• Why a definite number of atoms are used to constitute a particular molecule as for example, hydrogen molecule is H2 and not H3 or H4?
• Why do molecules have definite shapes?
• What is the nature of force existing between the combining atoms?
In this unit, we shall try to answer the above questions by studying the mode of formation interatomic bonds. Various theories of valency and interpretation of the nature of chemical have been related to the developments in the field of structure of atom, the electronic configuration of elements and periodic table.
In early days, the ability of various elements to combine with one another was expressed in terms of their valency. The concept of valency was not based on any logical understanding. The developments in the field of atomic structure provided foundations for various theories of valency. The initial contributions in this field came from W. Kossel and G. N. Lewis in 1916. They were the first to provide some logical explanation of valency which was based on the inertness of noble gases. This view, later on, came to be known as octet rule.
The rule was based on the electronic configuration of noble gases. All the noble gases (except helium), have 8 electrons in their outermost shell. Helium has 2 electrons in the outer shell. The noble gases are quite stable because they do not have any tendency to take part in chemical combination. The chemical inertness of noble gases was related to the presence of octet of electrons in their outermost shell because other elements which were chemically reactive had less than 8 electrons in their outermost shell. These observations led Kossel and Lewis to put forward a generalisation known as octet rule. The rule states that atoms of various elements enter
into chemical combination so as to attain the stable configuration of eight or two electrons in their outermost shell. They do so either by transference of electrons or by mutual sharing of electrons. The octet rule is quite useful in explaining the normal valencies of large number of elements. In the light of octet rule let us now study the formation of various bonds.
It was proposed that the electrons involved in the process of combination are outer shell electrons, therefore, these are called valence electrons. G .N. Lewis introduced the simple notations to represent the valence electrons in an atom. These notations are called Lewis symbols or electron dot symbols. According to these notations the symbol of the element represents the nucleus as well as the electrons in the inner shells. The electrons in the outer shell are represented by the dots surrounding the symbol. For example, chlorine atom has 7 electrons in the outermost shell. Its electronic configuration is 2, 8, 7. But chloride ion (Cl-) ion has configuration of 2, 8, 8. The Lewis symbols for chlorine atom and chloride ion has shown as follows:
The Lewis symbols for the elements of 2nd period are:
Significance of Lewis Symbols
The number of dots in the Lewis symbol represents the number of valence electrons. The common valencies of the elements can be calculated from the valence electrons. The common valency of the element is either equal to the number of dots o r 8 minus the number of dots. For example, the common valencies of Li, Be, B and C are 1, 2, 3 and respectively while those of N, 0, F and Ne are 8 minus number of dots, i.e., 3, 2, 1 and 0 respectively.