The electrostatic force of attraction that is strong enough to hold together two unlike charges, namely a cation and an anion is defined as the ionic bond.
In an ionic bond,
- A metal atom and a non-metal atom are involved.
- The metal atom loses electron/electrons, and a non-metal atom accepts the electron/electrons.
- A positively charged ion or cation is formed from a metal atom.
- A negatively charged ion or anion is formed from a non-metal atom due to the acceptance of electrons from metal atoms.
Formation of sodium chloride
In the formation of sodium chloride,
As can be seen, a sodium atom has one valence electron, which it needs to lose to attain the neon gas electronic configuration, chlorine has seven valence electrons, it needs one more electron to attain the argon configuration. Sodium atom transfers an electron to the chlorine atom, hence an ionic bond is formed.
The crystal lattice of NaCl reveals that the ions are arranged in a three dimensional pattern, the positive ion alternating with a negative ion.
Crystal lattice of NaClThe sodium chloride crystal is made up of ions. One sodium ion is surrounded by six chloride ions. One chloride ion is surround by six sodium ions. The crystal is cubic in shape.
Formation of magnesium oxide
A magnesium atom has two valence electrons, which it donates to the oxygen atom. Both magnesium and oxygen attain the neon configuration.
Formation of magnesium nitride
The electronic configuration of magnesium is 2,8,2 and that of nitrogen is 2,5.
A magnesium atom needs to lose two electrons, whereas the nitrogen atom needs to gain three electrons to attain the octet.
The chemical formula for magnesium nitride is Mg3N2.
Ion formation takes place as follows:
Each magnesium atom loses two electrons.
Each nitrogen atom gains three electrons.
Two magnesium atoms donate two electrons to each nitrogen atom while one magnesium atom donates one electron to each of the nitrogen atom. Hence, an ionic bond is formed. Magnesium and nitrogen, thus, attain stability by gaining the octet state.
The conditions necessary for formation of ionic compounds are:
a) It depends primarily on the formation of the cation and anion from neutral atoms.
b) When the difference in electro-negativities of the metal atom and non-metal is large, transfer of electrons take place.
c) A lower ionisation value facilitates easy formation of the cation. For example, the ionisation potentialof sodium is 5.1ev, whereas for chlorine it is 13.00ev. Hence, it is easy to remove electrons from a sodium atom.
d) If the electron affinity value is high, it facilitates formation of an anion.
For example, the higher value for chlorine is -3.80ev, so it forms an anion easily.
[Energy released = -3.80ev]
Characteristics of electrovalent compounds
1) Ionic compounds consist of ions and not molecules. For example, sodium chloride is an electrovalent compound consisting of equal number of sodium and chloride ions.2) Ionic compounds are hard crystalline solids. The crystal of sodium chloride has a definite shape. There is a strong electrostatic force of attraction, which holds the ions together. The ions cannot be easily separated. The crystals are hard and brittle.
3) Ionic compounds have high melting and boiling points. For example, sodium chloride has a high melting point of 1472oF and boiling point of 2575.4oF. Lithium chloride has a melting point of 1122.8oF and boiling point of 2462oF. Potassium chloride has a melting point of 1526oF and boiling point of 2732oF.As these compounds contain ions held together by strong electrostatic forces, very high amount of energy is required to overcome this force and break the crystal lattice. This explains the high melting point and boiling point of ionic compounds.
4) Electrovalent compounds or ionic compounds are usually soluble in water, but insoluble in organic solvents like ether, acetone, benzene, carbon disulphide and carbon tetrachloride. 'Like dissolves like': Ionic compounds dissolve in polar or ionic solvents.Water has a high dielectric constant. Water easily breaks the strong electrostatic force of attraction. The water molecule is polar in nature. The positively charged hydrogen atoms surround the anion. The negatively charged OH- (hydroxyl) surround the cation. Hence, the cation is separated from the anion, breaking the crystal lattice. The organic solvents contain non-polar molecules. Therefore, they are unable to break the electrostatic forces of attraction.
5) Ionic compounds conduct electricity when dissolved in water. They also conduct electricity when melted. They do not conduct electricity in the solid state. In the solid state, the ions are held together in fixed positions by the electrostatic forces of attraction. Therefore, the ions are not free. When dissolved in water, the ions are separated. The water molecules break the strong electrostatic force of attraction. This results in the formation of free mobile ions. To conduct electricity, free mobile ions are required. In the molten state too, the ions are separated from each other. The strong electrostatic force of attraction is broken while melting resulting in the formation of free ions.
6) Ionic compounds have a high density. They are generally heavier than water. The oppositely charged ions in an ionic compound are held very closely by strong electrostatic forces of attraction, resulting in the number of ions per unit volume increases. Consequently, increasing in mass per unit volume of the compound.
7) Ionic compounds are involved in ionic reactions. The ionic reactions occur very fast as ions react quickly in solution.8) Size of the cation involved in bonding is smaller compared to the neutral atom. A cation loses electrons, so there is a reduction in the number of shells. Hence, the size of the ion becomes smaller. On the other hand, in an anion, there is an increase in the number of electrons in a particular shell. This leads to a decrease in the effective nuclear charge. The protons feel a greater attraction towards the electrons, therefore the size is reduced compared to the atom.