Question

The strongest hydrogen bond is present in
A. H-F……H-F
B. H-Cl……H-Cl
C. H-O…….H-O
D. H-N…….H-N

Answer 1

Hint: Hydrogen bond is a bond which is governed by the key point of electronegativity difference between the atoms forming the bond. More the difference in electronegativity, more is the strength of hydrogen bonds.


Complete step by step solution:
-There are mainly 4 types of bonds – ionic, covalent, hydrogen and vander-waal forces of interaction. Ionic bonds are formed by losing/gaining electrons. Covalent bonds are formed by sharing of electrons. Hydrogen bonds are formed by the interaction of lone pairs of electrons with another atom. Vander-waal forces are physical forces which are dipole-dipole interactions.
-To understand hydrogen bonds, we need to first see what electronegativity is. Electronegativity is actually the tendency of atoms to attract the electrons which are shared in a bond, towards itself. More is the electron affinity of an atom towards an electron, more is its electronegativity.
-Along the period, the atomic number increases and so does the number of electrons without altering the number of shells of atoms. Thus, the electronegativity increases across the period. This can be explained on the basis of increase in ionization energy also.
-Down the group, the number of shells increases but the valence electrons remain the same. Atomic size increases down the group. This increases the distance between the nucleus of an atom to the valence electrons. Thus, the electronegativity decreases down the group.
-Electronegativity is found by Pauling scale and on that scale, the electronegativity is maximum for F. Hydrogen bond is a bond which occurs between the lone pairs of an electronegative atom and the hydrogen atom. The electronegative atom attracts the electron cloud of hydrogen atom thus generating a partial positive charge on hydrogen atom and partial negative charge on itself.
-Since the strength of hydrogen bond is governed by the electronegativity of an atom, we can conclude that hydrogen bond will be the strongest in the compounds formed by hydrogen and fluorine atoms. Actually, only three elements can show hydrogen bonding. They are N, O and F.
-In the given options, we see that 3 of the given compounds can show hydrogen bonding but the strongest bond will be observed in hydrogen fluoride.


Therefore, the correct option is A.


Note: Nitrogen and chlorine have the same value of electronegativity which is 3.0. Theoretically, chlorine has more electronegativity but when we see the reactions of these compounds, we find that the electronegativity of nitrogen is more than that of chlorine. Still, theoretically we write chlorine has more electronegativity.