Hello friends, welcome to our blog. Today we are going to discuss ionic bond. An ionic bond is also called as electrovalent bond.
How do ionic bonds form
In the formation of molecules only electrons present in outermost shell or valence shell of atom normally participate while the electrons belonging to inner energy shell are mostly not involved in the formation of bond.
An ionic bond is formed when there is complete the transfer of one or more electrons from one atom to another.
Bond between metal and nonmetalIt is generally formed between the metals and non-metals.The metal atom loses one or more electrons present in its valence shell and these electrons are taken up by the non-metallic atom.
Since, the atoms are electrically neutral (electrons and proton are equal in number),by losing electrons,the metal atom changes to positive ion or cation.Similarly, the non-metal atom gaining the electrons, get converted to negative Ion or anion. The opposite charged ions are attracted towards each other and therefore, come closer resulting the formation of ionic bond (electrovalent Bond).Thus, ionic or electrovalent bond may be defined as :
The coulombic or electrostatic force of attraction which hold the opposite charged ions together.
Formation of sodium chloride (NACl). Sodium atom (Z=11) has one valence electron(2, 8, 1). Similarly chlorine atom (Z=17) has a 7 valance electron (2,8,7). Sodium atom loses its only valence electron and changes Na+ ion(cation). This electron is taken up by chlorine atom which changes to a Cl- ion(anion).Both of these ions are mutually attracted to form NaCl molecule.
Na + ion has the configuration of Ne while Cl- ion represent configuration of Ar. Both are inert gas atoms.These the atoms take part in chemical combination or bond formation in order to complete the octet( 8 electron in valance shell) and achieve the electronic configuration of nearest noble gas atom.
Formation of calcium fluoride(CaF2 ).Calcium atom (Z=20) has two valence electron (2,8,8,2). while fluorine atom (Z=9) has seven valence electron in its valence shell (2,7).Now,Ca atom wants to get rid of both valence electrons but each fluorine atom is in position to take up one electron only, This means that these electrons are accepted by two fluorine atoms as shown below.
Ca 2 + ion has electronic configuration of inert gas argon while F- ion has the electronic configuration of neon.
It is defined as the number of electrons which an atom lose or gain in the formation of ionic bond.
Actually, electrovalency of an atom is in number of unit charges(positive or negative) on the ion in an ionic bond. If the unit charges are positive, electrovalency is positive. Similarly, electrovalency is negative in case these are the negative. for example in CaF2 electrovalency of calcium is 2(positive) and the fluorine is 1 (negative)
Factor influencing the ionic bond formation
We have learnt that in bond formation one atom loses one or more electrons while the other accept them. As a result cation and anion formed .They mutually attracted by the electrostatic force of attraction result the formation of ionic bond. According to Kossel's scientist ionic bond depends upon the following factors:
The amount of energy required to remove the loosely bound electron from an atom is called ionization enthalpy.
Lesser the ionization enthalpy required easy form of cation.The alkali metals and alkaline earth metals present in s- block normally form cation since they have comparatively low ionization enthalpy.
Electron gain enthalpy
The electron released in the formation of cation are to be accepted by the other atoms taking part in the ionic bond formation.The electron accepting tendency of an atom depends upon electron gain enthalpy. It may be defined as:
energy released when an isolated (gaseous) atom take up electron to form anion.
Greater the negative electron gain enthalpy , easier will be the formation of anion. The Halogen present in group 17 have the maximum tendency to form anions as they have very high negative electron gain enthalpy.
The members of group 16 oxygen family such as Oxygen also forms in anion. It has lesser negative electron gain enthalpy then group 17 because it has lesser tendency to form anion.
Lattice energy or enthalpy
The ionic compound exist as crystalline solid and arrangement is called as crystal lattice. Since the ions of charged species, energy (known as lattice energy) is released in the attraction of ions so it may define as:
Energy released when 1 mole of crystalline solid is formed by the combination of oppositely charged ions.
Greater the magnitude of lattice energy more will be the stability of ionic bond. Ionic compounds
the lattice energy depends upon the following factors:
Size of ion:- The size of ion influence the lattice energy. Smallest size lesser will be the internuclear distance and thus, greater will be the lattice energy. For example: lattice energy of NaCl is more than KCL because the radius of Na + is smaller as compared to K+ ion.
Charge on the ion:-Greater the magnitude of charge on ions higher will be the inter ionic attraction and thus,greater will be the higher value of lattice energy.
So, we conclude that if the magnitude of lattice energy and the negative electron gain enthalpy is greater than that of the ionization enthalpy required, a stable chemical bond will be formed in case it is less than the bond will not form.
General characteristics of ionic bond
1. Melting and boiling point: Because of strong inter ionic forces the ionic compound have generally high melting and boiling point.
2.Solubility: Ionic compounds generally dissolve in polar solvent (like water). Actually in such solvent the polar(water) molecule interact with the ions of crystalline solid(NaCl salt) energy is released. Then the energy released is known as hydration energy and the energy released overcome(weakened) electrostatic force between ions thus get separated and go into the solution. The ionic compound do not dissolve in organic solvent like Benzene carbon tetra chloride because they are non polar in nature.
3.Ionic reactions :The ionic compound the reacts each other through ions which are formed in the solution. Therefore,these take part in ionic reaction which are very fast and do not required any specific condition. For example aqueous solution of NaCl and AgNO3 are mixed a white precipitate of AgCl is immediately formed.