Combination of Elements
Atoms of elements combine to form molecules of compounds. This combination can occur in two ways: either by electrovalent bonding or covalent bonding. In all chemical reactions it is the electrons from the outermost shell of an atom that are involved in interacting with other atoms, either by their transfer or by sharing.
Electrovalent Bonding
When an atom donates one, two or three electrons from its valence shell to another atom, which has the ability to accept these electrons, it is known as electrovalency. As a result of electrovalency, both these atoms achieve the structure of an inert gas.
Electrovalent Compounds
Electrovalent compounds are compounds formed by completed transfer of electrons from a metallic atom to a non-metallic atom resulting in the formation of cation and anion.
Covalent Bonding
When the combining atoms have 4, 5, 6, or 7 electrons in the outermost or valence shell, they cannot donate electrons. Instead, they mutually share electrons in order to complete their octet in the outermost orbit or shell. Such a type of valency is referred to as covalency and the bond created between such atoms is called a covalent bond. The compounds so formed are called covalent compounds or molecular compounds.
Properties of Electrovalent and Covalent Compounds
Comparison of Properties of Electrovalent and Covalent Compounds
| Property | Electrocalent or ionic compounds | Covalent or molecular compounds |
|---|---|---|
| Structure of charged ions | They consist of oppositely charged molecules | They consist of uncharged or neutral molecules |
| Physical state and hardness | The inter-atomic attraction is high) hence they are brittle, hard) crystalline solids | Due to weak inter atomic attraction, they are usually liquids or gasesj those which are solids are soft and waxy |
Polar and Non-polar Molecules
When compounds are formed from elements whose atoms have different electronegativities, the electrons forming the bonds will have a tendency to be attracted closer to the more electronegative element. In such instances covalent molecules develop some ionic character and are called polar molecules. As a result of polarity, a small negative charge (d-) appears on the more electronegative atom and a small electropositive charge (d+) appears on the less electronegative atom.
Oxidation and Reduction
In most of the combinations discussed above, it can be seen that many reactions involve transfer of electrons from one species to another. These electron transfer reactions can also be studied under another category of reactions called the oxidation and reduction reactions.
Oxidation
"Oxidation is the loss of electrons from an atom."
Oxidation reactions
Sodium atom loses an electron and gets converted to a positively charged sodium ion. Sodium atom is oxidized to sodium ion.
Reduction
"Reduction is the process in which an atom gains one or more electrons."
Reduction reactions
A chlorine atom will gain an electron to become a chloride ion.
Hydronium and Ammonium Ions
The hydronium ion is a hydrogen ion in association with a molecule of water. In a water molecule, the oxygen atom has two pairs of unshared electrons in its outermost shell. The hydrogen ion does not have any electron in its valence shell. Hence it shares a pair of electrons from the oxygen atom by means of co-ordinate co-valency to form a hydronium 
Summary
Electrovalent bond is the attractive force between the oppositely charged ions, which comes into existence by the transference of electrons.
