The force that holds the atoms together in a molecule of a substance is known as chemical bond. In 1916, Kossel put forward the theory of bonding based on the electronic concept of the atom. According to his theory, the formation of bond depends upon the number of electrons present in the outermost shell of an atom. He gave some postulates to support his theory. He observed that atoms of noble gases are stable. They have 8 electrons (except helium which contains 2 electrons) in the outermost shell. On this basis he gave octet rule which states that, for an atom to be stable it must possess eight electrons in its outermost shell. Chemical bond formed by the complete transfer of electrons from one atom of an element to one atom of another element is known as electrovalent bond or ionic bond. Most of the ionic compounds are formed when elements belonging to Group 1 and 2 of periodic table combine with elements of Group 16 and 17. Electrovalent compounds consist of charged particles (cation and anion). They are crystalline, hard and brittle solids having high melting and boiling points. They are soluble in water but insoluble in non polar solvents. The cation and anion in these compounds are held together tightly by electrostatic force of attraction. They conduct electricity in their molten state and in aqueous solution.
Sodium atom is silvery white in colour while sodium cation isMarks:1
Sodium cation (Na+) is colourless.
During the formation of calcium oxide, calcium atomMarks:1
loses two electrons.
Calcium atom acquires a stable configuration by losing two valence electrons. Oxygen requires 2 electrons to complete its octet. Calcium atom loses its 2 valence electrons and transfers to oxygen atom. As a result, the calcium atom forms a calcium ion (Ca2+) and oxygen atom forms an oxide ion (O2). These ions combine to form calcium oxide.
A chemical bond formed by the complete transfer of one or more electrons from one atom to other is known asMarks:1
An electrovalent bond is formed by the complete transfer of electrons from one atom to another forming cation and anion respectively. These oppositely charged ions attract each other and form a chemical bond.
Ions are the constituent particles ofMarks:1
Electrovalent compounds are made up of ions whereas covalent compounds are made up of atoms.
If the number of electrons presents in an atom are two more than the nearest noble gas, it will formMarks:1
An atom will acquire a stable configuration of the nearest noble gas by either losing 2 electrons or by gaining 6 electrons. Since gaining of 6 electrons to complete its octet is difficult. Therefore, it loses 2 electrons to attain stability. Hence, it will form a cation.