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It is common knowledge that anything that has mass and can occupy space is called matter. Matter is made up of elements of more than one type. No element except the noble gases can exist independently in nature. They usually occur as a group of atoms. There is some sort of force holding the group of atoms together known by the name of chemical bonds. It is not necessary that a chemical bond is formed between similar atoms. Mostly different atoms combine through chemical bonds. In 1916, Kössel and Lewis explained the formation of chemical bonds in terms of electrons.
Lewis Approach
Lewis described the atom in terms of a kernel, with the outer shell having a maximum of 8 electrons. The term kernel is used to describe the nucleus along with the inner electrons. He presumed that the outer 8 electrons will occupy the corner of the cube surrounding the kernel. All the eight corners will be occupied by the noble gases. This stable arrangement is called the octet of electrons. Lewis postulated that when atoms are linked by a chemical bond, they achieve a stable octet. In the case of NaCl, this takes place by electron transfer from Na to Cl. In the case of molecules like Cl2 or H2,this happens by the sharing of electrons. Each atom ends up having a stable outer octet of electrons.
Lewis Symbols
Lewis introduced a notation for representing valence electrons in an atom. These are called Lewis Symbols. The number of dots surrounding the symbol determines the number of valence electrons. These can be used to determine group valence of an element, which is generally equal to the number of dots in Lewis Symbols or 8 – number of dots in Lewis Symbols. So, Li is shown having 1 dot, B has 3 dots, Ne has 8 eight dots. This means that the group valence of Li is 1, group valence of B is 3, whereas group valence of Ne is 8 – 8 = 0.
Kössel Postulates
In relation to chemical bonding, Kössel gave the following postulates known as kössel’s theory of chemical bonding :
- In the periodic table, noble gases separate the highly electronegative halogens from the highly electropositive alkali metals.
- When a halogen atom gains electrons, it forms an anion. When an alkali metal atom donates an electron, it forms a cation.
- Both the negative and positive ions, i.e. anions and cations, attain a noble gas configuration. Only helium (He) has a duplet of electrons. Apart from this, every noble gas has a particularly stable configuration of an octet of electrons, ns2np6.
- These negative and positive charges are stabilised by the electrostatic attraction between the oppositely charged particles.
The following example can elaborate on it.
Ca has a configuration of [Ar]4s2. It is a group 2 element and can easily lose 2 electrons to attain the configuration of argon.
F has a configuration of [He] 2s22p5
It is a group 17 element and can gain an electron to attain the stable electron configuration of neon. So, the 2 electrons lost by Ca can be gained by 2 F atoms.
Ca→ Ca2+ + 2e–
F + e– → F–
Ca2+ + 2F– → CaF2
The bond formed due to electrostatic attraction between the positive cation and negative anion is termed an electrovalent bond. We can say that the number of unit charge(s) on the ion is equal to the electrovalence and that Ca has an electrovalence of +2, while F has electrovalence of -1.
Importance of Kössel Postulates
The importance of Kössel Postulates can be understood from the fact that it provides the abscess for modern concepts concerning the formation of ions by transfer of electrons and the formation of ionic crystalline compounds.
His views also helped in the systematisation and understanding of ionic compounds.
It is also to be noted that he was aware that there are still a large number of compounds that did not fit in this concept.
Conclusion
Except for the noble gases, which are capable of independent existence, most of the elements combine to form compounds. Kössel and Lewis were the first scientists to provide an explanation as to why chemical bonds are formed. They explained chemical bonds in terms of electrons.
Lewis gave a notation of writing symbols of elements. He imagined atoms to be made up of a kernel and outer shell with a maximum of 8 electrons. The term kernel was used to describe the nucleus along with inner electrons.He presumed that the outer 8 electrons will occupy the corner of the cube surrounding the kernel. All the eight corners will be occupied in noble gases. This stable arrangement was called octet of electrons.
In his postulates. Kössel explained that an electropositive element loses electrons and an electronegative element gains electrons to achieve a stable electronic configuration of noble gases.
The chemical bond formed between the negative anion and positive cation is called electrovalent bond because of electrostatic attraction present between them. So we can say that the number of unit charge(s) on the ion is equal to the electrovalence.
The importance of Kössel Postulates can be understood from the fact that it provides the abscess for modern concepts concerning the formation of ions by transfer of electrons and the formation of ionic crystalline compounds.
