A chemical bonding is a force that acts between two or more atoms to hold them together as a stable molecule. Atoms of elements other than noble gases have unstable electronic configuration and their outermost shell is incomplete. They can gain, lose or share electrons to attain a stable electronic configuration of the nearest noble gas.
In this lesson we are going to cover:
For an atom to achieve stable electronic configuration, it must have -
Thus the chemical composition of atoms involves the redistribution of electrons so as to achieve a stable electronic configuration. They tend to attain stable electronic configuration of the nearest noble gas by:
The formation of an electrovalent compound involves the transfer of valence electrons from one atom(generally metallic) to another atom(generally non-metallic).
Metallic atom - loses electrons and becomes cation, X − 1e−→ X1+
Non-metallic atom - gains electrons and becomes an anion, Y + 1e− → Y1−
As ions are opposite charged particles, they attract one another to form an electrovalent compound.
Example1: Sodium Chloride (NaCl)
Electronic configuration of Sodium atom[Atomic number 11] - 2, 8, 1
Electronic configuration of Chlorine atom[Atomic number 17] - 2, 8, 7
Sodium atom attains stable electronic configuration of nearest noble gas - Neon by losing one electron from its valence shell and becomes a positively charged ion Na1+. Chlorine atom attains stable configuration of the nearest noble gas - Argon by gaining one electron in its valence shell and becoming negatively charged ion Cl−.
Na − 1e−→ Na1+
[2, 8, 1] [2, 8]
Cl + 1e− → Cl1−
[2, 8, 7] [2, 8, 8]
Na + Cl ⇒ Na1+Cl1− ⇒ NaCl
Example 2: Magnesium Chloride (MgCl2)
Electronic configuration of Magnesium atom[Atomic number 12] - 2, 8, 2
Electronic configuration of Chlorine atom[Atomic number 17] - 2, 8, 7
Magnesium atom attains stable electronic configuration of nearest noble gas - Neon by losing two electrons from its valence shell and becomes a positively charged ion Mg2+. Chlorine atom attains stable configuration of the nearest noble gas - Argon by gaining one electron in its valence shell and becoming negatively charged ion Cl−.
To accept the two electrons of Mg there are two chlorine atoms.
Mg − 2e− ⇒ Mg2+, 2Cl + 2e−⇒ 2Cl−
Mg + 2Cl ⇒ Mg2+ 2Cl1− ⇒ MgCl2
In covalent bonding there is mutual sharing of electrons between two pairs of atoms of non-metallic elements and the compound thus formed is called a covalent compound. Electrons in the valence shell are mutually shared by the atoms of each element such that each atom acquires a stable electronic configuration. Bond is single [-], double[=] or triple[ = ] covalent.
Example 1: Oxygen [O2]
The oxygen atom[Atomic number 8, electronic configuration 2, 6] needs two electrons to attain a stable octet structure. Each of the O atoms contributes two electrons so as to have two shared pairs of electrons between them resulting in the formation of a double covalent bond, O = O.
Example 2: Methane [CH4]
One atom of carbon shares four electron pairs - one with each of the four atoms of hydrogen.
Polar and Non-Polar Covalent Compounds
| Non-Polar Covalent Compounds | Polar Covalent Compounds |
| Covalent compounds are said to be non-polar when shared pair of electrons are equally distributed between the two atoms. | Covalent compounds are said to be polar when shared pair of electrons are unequally distributed between the two atoms. |
| No charge separation takes place. The covalent molecule is symmetrical and electrically neutral. |
Charge separation takes place. The atom which attracts electrons more strongly develops a slight negative charge. |
| Example: H2, Cl2, O2, CH4 | Example: H2O, NH3, HCl HCl: As the chloride ion is more electronegative than hydrogen ion, so chloride ion carries partially negative character while hydrogen carries partial positive character.  |
Properties and comparison of Electrovalent and Covalent Compounds
| Electrovalent Compound | Covalent Compound |
| Compounds are formed by the transfer of electrons between atoms. | Compounds are formed by sharing of electrons between atoms. |
| Formed as a result of the large difference in electronegativity of atoms. | Formed as a result of a small difference in electronegativity of atoms. |
| Hard, crystalline solids. | Usually liquids or gases. |
| Reactions are fast and rapid. | Reactions are slow. |
| They can conduct electricity in a molten or solution state. | Covalent compounds cannot conduct electricity. |
| Have high melting and boiling point. | Have low melting and boiling point. |
| Ions are involved in bond formation. | Atoms are involved in bond formation. |
