Question

Covalent bonded compounds have low melting and boiling point because of :

• A. Sharing of an electron pair between two atoms.
• B. Weak intermolecular forces.
• C. Attraction between molecules is very large.
• D. None of these

Hint:

Always distinguish between intramolecular forces (chemical bonds within a molecule) and intermolecular forces (forces between molecules).
Low boiling point is a feature of molecular substances with weak intermolecular forces.

Answer 1

The Correct Option is B

Step 1: Separate the two kinds of forces at play.
Inside a covalent molecule, atoms are held by strong shared electron pair bonds, but between one molecule and its neighbour there is a completely separate, much weaker pull called an intermolecular force.
Step 2: Ask what actually breaks when a substance melts or boils.
Melting or boiling does not snap the covalent bonds inside the molecule, it only needs to pull the molecules apart from each other, so it is the weak intermolecular attraction that has to be overcome, not the strong bond within the molecule.
Step 3: Compare with ionic solids for contrast.
In an ionic solid like common salt, the entire crystal is held together by strong electrostatic attraction between charged ions, so a huge amount of energy is needed to melt it. Covalent molecular substances have nothing that strong holding the molecules together, only weak van der Waals or dipole attractions.
Step 4: Draw the conclusion.
Because so little thermal energy is required to separate the molecules, covalent compounds generally melt and boil at low temperatures. \[ \boxed{\text{Low melting and boiling points arise from weak intermolecular forces}} \]