Atoms having 8 electrons in their outermost shell are very stable and they did not form compounds. Other atoms such as hydrogen, sodium, chlorine etc. which do not have 8 electrons in their outermost shell undergo chemical reactions. They can stabilize by combining with each other and attain the above configurations of noble gases i.e. 8 electrons (or 2 electrons in case of helium) in their outermost shells.
Thus, atoms tend to attain a configuration in which they have 8 electrons in their outermost shells. This is the basic cause of chemical bonding. This attainment of eight electrons for stable structure is called the octet rule.
Atoms are held together in compounds by the forces of attraction which result in formation of chemical bonds. The formation of chemical bonds results in the lowering of energy which is less than the energy the individual atoms. The resulting compound is lower in energy as compared to sum of energies of the reacting atom/molecule and hence is more stable.
The chemical bond formed by transfer of electron from a metal to a non-metal is known as ionic or electrovalent bond.
For example, when sodium metal and chlorine gas are brought into contact, they react violently and we obtain sodium chloride.
2Na (s) + Cl2 (g) → 2NaCl (s)
Sodium (Na) has the atomic number 11 and we can write its electronics configuration as 2,8,1 i.e. it has one electron in its outermost (M) shell. If it loses this electron, it is left with 10 electrons and becomes positively charged. Such a positively charged ion is called a cation. The cation in this case is called sodium cation, Na+.
Sodium cation has 11 protons but 10 electrons only. It has 8 electrons in the outermost (L) shell. Thus, sodium atom has attained the noble gas configuration by losing an electron present in its outermost shell. Loss of electron results into formation of an ion and this process is called ionization. Thus, according to octet rule, sodium atom can acquire stability by changing to sodium ion (Na+).
Chlorine atom having the atomic number 17, has the electronic configuration 2,8,7. It completes its octet by gaining one electron from sodium atom (at. no. 11) with electronic configuration 2,8,1.
Both sodium ion (Na+) and chloride ion (Cl–) combine together by ionic bond and become solid sodium chloride (NaCl).
The chlorine atom has gained an additional electron hence it has become a negatively charged ion (Cl–). Such, a negatively charged ion is called an anion. Chloride ion has 8 electrons in its outermost shell and it therefore, has a stable electronic configuration according to the octet rule.
Since the cation (Na+) and the anion (Cl–) formed above are electrically charged species, they are held together by Coulombic force or electrostatic force of attraction. This electrostatic force of attraction which holds the cation and anion together is known as electrovalent bond or ionic bond.
Since the ionic compounds contain ions (cations and anions) which are held together by the strong electrostatic forces of attraction, they show the following general characteristic properties:
Ionic compounds are crystalline solids. In the crystal, the ions are arranged in a regular fashion. The ionic compounds are hard and brittle in nature.
Melting and boiling points
Ionic compounds have high melting and boiling points. The melting and boiling points of ionic compounds are high because of the strong electrostatic forces of attraction present between the ions. Thus, it requires a lot of thermal energy to overcome these forces of attraction.
Ionic compounds conduct electricity in their molten state and in aqueous solutions. Since ions are free to move in the molten state, they can carry current from one electrode to another in a cell.
In aqueous solution, water is used as a solvent to dissolve ionic compounds. It weakens the electrostatic forces of attraction present among the ions. When these forces are weakened, the ions become free to move, hence they can conduct electricity.
Ionic compounds are generally soluble in water but are insoluble in organic solvents such as ether, alcohol, carbon tetrachloride etc. However, a few ionic compounds are insoluble in water due to strong electrostatic force between cation and anion. For example barium sulphate, silver chloride and calcium fluoride.
A bond, which is formed by sharing of electrons between the atoms is called a covalent bond.
In formation of hydrogen molecule (H2), the hydrogen atom has one electron. It can attain the electronic configuration of the noble gas helium by sharing one electron of another hydrogen atom. When the two hydrogen atoms come closer, there is an attraction between the electrons of one atom and the proton of another and there are repulsions between the electrons as well as the protons of the two hydrogen atoms.
In the beginning, when the two hydrogen atoms approach each other, the potential energy of the system decreases due to the force of attraction. The value of potential energy reaches a minimum at some particular distance between the two atoms. If the distance between the two atoms further decreases, the potential energy increases because of the forces of repulsion. The covalent bond forms when the forces of attraction and repulsion balance each other and the potential energy is minimum.
H. + .H → H : H → H2
The oxygen atom has atomic number 8. It has 8 protons and also 8 electrons. The electronic configuration of oxygen atoms is 2,6. Now each oxygen atom needs two electrons to complete its octet. The two oxygen atoms share two electrons and complete their octet.
The 4 electrons (or 2 pairs of electrons) which are shared between two atoms of oxygen are present between them. Hence these two pairs of shared electrons can be represented by two bonds between the oxygen atoms.
The two oxygen atoms are said to be bonded together by two covalent bonds. Such a bond consisting of two covalent bonds is also known as a double bond.
The atomic number of nitrogen is 7. Thus it has 7 protons and 7 electrons present in its atom. The electronic configuration can be written as 2,5. To have 8 electrons in the outermost shell, each nitrogen atom requires 3 more electrons. Thus, a sharing of 3 electrons each between the two nitrogen atoms is required.
Each nitrogen atom provides 3 electrons for sharing. Thus, 6 electrons or 3 pairs of electrons are shared between the two nitrogen atoms. Hence, each nitrogen atom is able to complete its octet.
A bond which consists of three covalent bonds is known as a triple bond.
The covalent compounds consist of molecules which are electrically neutral in nature. The forces of attraction present between the molecules are less strong as compared to the forces present in ionic compounds. Therefore, the properties of the covalent compounds are different from those of the ionic compounds.
Because of the weak forces of attraction present between discrete molecules, called intermolecular forces, the covalent compounds exist as a gas or a liquid or a solid. For example O2, N2, CO2 are gases; water and CCl4 are liquids and iodine is a solid.
Melting and Boiling Points
As the forces of attraction between the molecules are weak in nature, a small amount of energy is sufficient to overcome them. Hence, the melting points and boiling points of covalent compounds are lower than those of ionic compounds.
The covalent compounds contain neutral molecules and do not have charged species such as ions or electrons which can carry charge. Therefore, these compounds do not conduct electricity and are called poor conductors of electricity.
Covalent compounds are generally not soluble in water but are soluble in organic solvents such as alcohol, chloroform, benzene, ether etc.