Chemistry [5 Mark Question Answer]

The necessary conditions for the formation of covalent molecule are:
Number of valence electrons: Both the participating atoms should have four or more valence electrons in their valence shell.
Equal electro negativities: The combining atoms should have equal electro negativities so that no transfer of electrons takes place.
Equal electron affinities: The combining atoms should also have equal electron affinities i.e. equal attraction for electrons.
Ionization energy: It should be high for both the atoms so that there is no chance of removal of electrons.
High nuclear charge and small inter nuclear distance: Both these conditions favor the formation of covalent bond because during the formation of a covalent bond the electron density gets concentrated between the nuclei of the combining atoms and this electronic charge is responsible for holding the two nuclei together.
The properties of covalent compounds are:
Nature: They are generally volatile liquids or gases. Some may be gases like urea, sugar etc.
Low melting and boiling points: Since the intermolecular forces of attraction are weak, very small amount of heat energy is required to overcome these forces hence their melting and boiling points are low.
Electrical conductivity: Since covalent compounds are made up of molecules and not ions, so they do not conduct electricity.
Solubility: These are insoluble in water but soluble in organic solvents.
Ionization in solution: These do not ionize when dissolved in water except some polar covalent compounds like HCl.
Molecular reactions: These participate in reactions as a molecule so the reactions are called molecular reactions. These are slow reactions.

Hydrogen chloride has a polar covalent bond because in hydrogen chloride the higher electronegativity of chlorine atom attracts the shared electron pair towards itself. As a result, the chlorine atom gets a partial negative charge while the hydrogen atom gets a partial positive charge. Hence such a covalent bond with charge separation is called polar covalent bond. While methane has a non polar covalent bond because in case of methane molecule the shared electron pairs are at equal distance from the carbon and hydrogen atoms, because neither the carbon atom nor the hydrogen atom has enough electronegativity difference between each other to attract the shared pairs of electrons towards itself. Hence no charge separation occurs in the covalent bond due to which it is called non polar covalent bond.
Concept Insight : When a covalent bond is formed between the atoms of the same elements of equal electronegativity then the electron pairs are shared equally between the atoms and the bond so formed is called non polar covalent bond. On the other hand, if the covalent bond is formed between atoms of different elements, with difference in electro negativity, the electrons are not shared equally between the atoms. The more electronegative atom pulls the bonded pair of electrons towards itself and acquires negative charge while the other less electro negative atom acquires positive charge and the bond becomes polar covalent bond.