Questions & Answers

The hydrides of the first element in the group 15-17, namely \[N{H_3},{\text{ }}{H_2}O\] and \[HF,\] respectively show abnormally high values for melting and boiling points. This is due to:
A.Small size of N, O , and F
B.The ability to form extensive intermolecular H- bonding
C.The ability to form extensive intramolecular H- bonding
D.Effective van der Waals interaction

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Hint: Hydrogen bonding interaction involves a hydrogen atom located between a pair of other atoms having a high affinity for electrons.

Complete step by step answer:
-The above reason is due to the ability to form extensive intermolecular H-bonding.
-This is because of the small size and high electronegativity of elements, have the ability to form extensive intermolecular (i.e. between two molecules) hydrogen bonding
-Thus, a large amount of energy is required to break these bonds i.e. the melting and boiling points of hydrides of these elements are abnormally high.
-The interaction between molecules is much stronger when there are intermolecular hydrogen bonds because the bonds are formed between molecules.
-Further, the strength of hydrogen bond depends upon the coulombic interactions between the electronegativity of the attached atom and hydrogen.
-It is a special type of dipole-dipole attraction between molecules.
Intermolecular hydrogen bonding is as shown:

Hence, option B is correct.

Dipole- dipole interactions are the strongest intermolecular interactions and the ionic bond is the weakest of the true chemical bonds that bind atoms to atoms.