Boiling points of NH3 and HF are abnormally high as compared to those of hydrides of other elements in their respective groups ?
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The boiling points of the trihydrides increase as one moves down the group. The abnormally high boiling point of ammonia is due to the association of the molecules due to hydrogen bonding. A hydrogen bond is the attractive interaction of a hydrogen atom and an electronegative atom, such as nitrogen, oxygen or fluorine, that comes from another molecule or chemical group. Intermolecular hydrogen bonding is responsible for the high boiling point of water (100 °C) compared to the other group 16 hydrides that have no hydrogen bonds.
The electronegativities of P, As and Sb (2.1, 2.0 and 1.9 respectively) are so close to that of hydrogen (2.1) that no hydrogen bonding is present. However, the increasing trend of boilings points is due to heaviness of the molecules only. This "heaviness" basically allows for more van der waal forces. In general, the heavier the molecule, the stronger the van der Waal's force of interaction. The reason for this interactions is the temporary attraction of the negatively-charged electrons of one atom to the positive-charged nucleus of another, thus forming this bond. Id-id is affected by the size of molecules simply because of this. How? The larger the size of any molecule, the more the electrons it will have, therefore increasing the probability of attraction between the electrons of one atom and the nucleus of another. So, therefore with an increase in the size of a molecule, its polarizability also increases and thus the strength of this id-id interactions increases as a result.
The electronegativities of P, As and Sb (2.1, 2.0 and 1.9 respectively) are so close to that of hydrogen (2.1) that no hydrogen bonding is present. However, the increasing trend of boilings points is due to heaviness of the molecules only. This "heaviness" basically allows for more van der waal forces. In general, the heavier the molecule, the stronger the van der Waal's force of interaction. The reason for this interactions is the temporary attraction of the negatively-charged electrons of one atom to the positive-charged nucleus of another, thus forming this bond. Id-id is affected by the size of molecules simply because of this. How? The larger the size of any molecule, the more the electrons it will have, therefore increasing the probability of attraction between the electrons of one atom and the nucleus of another. So, therefore with an increase in the size of a molecule, its polarizability also increases and thus the strength of this id-id interactions increases as a result.
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