Why the elements of 17 are called halogens?
The name halogen is derived from Greek word ‘halos’ and ‘gens’ meaning sea salt producer. Chlorine, Bromine and Iodine occur as salts in sea water.
Why halogen molecules are coloured?
All halogens are coloured. Fluorine is pale yellow, chlorine is greenish yellow, and bromine is reddish brown while iodine is shining grey solid at room temperature. Vapours of iodine are violet. Their colour is due to the fact that the molecules absorb visible light radiations. Thus results in the excitation of valence electrons to higher energy levels. Smaller the size of halogen, greater is the energy required for excitation. Fluorine, due to small size absorbs violet radiations and appears yellow. Iodine absorbs lower energy and appears violet.
The oxidation state of fluorine is only -1 but other members of group17 exhibit -1, +1, +3, +5 and +7 oxidation state.
The reason is that fluorine is most electronegative and cannot exhibit positive oxidation state. The higher oxidation states for fluorine are not possible due to the non-availability of d-orbital’s.
Which is the weakest halogen acid and why?
Out of HF, HCl, HBr and H-I, the weakest acid is HF. The reason is that HF exists in the form of associated molecules by hydrogen bonding. Also the dissociation energy of HI is least. Thus, HI is the strongest and HF is the weakest acid.
Fluorine is more reactive than any other halogen. Explain.
The high reactivity of fluorine is due to the following factors:
(i)                  Low dissociation energy. The dissociation energy of fluorine is lower as compared to chlorine and F2 readily dissociates to Fluorine atom.
(ii)                High electro affinity. It has high value of electron affinity and readily from an anion.
(iii)               Small size. Due to its small size, it can attack central atom easily and thus, the central atom can exhibit maximum oxidation state.
Why HF is least volatile whereas HCl is most volatile?
HF shows the property of hydrogen bonding and exists in the form of associated molecules. Due to this, its boiling point is relatively high and it is least volatile. Hydrogen bonding is absent in order halides, and their boiling points depends upon the magnitude of van der Waal’s force. HCl has least Vander Waal’s force and is most volatile.
Why halogens have high value of electro negativity?
It is due to their small size and high effective nuclear charge. Electron negativity of group 17 elements decrease down the group. Fluorine is the most electro negativity element.
Why pure HI solution in a bottle acquires brown colour after some time?
Oxygen of air oxidises HI to water and iodine. The liberated iodine imparts brown colour to the solution.
                                                    4HI+O2ž2H2O+2I2
Fluorine cannot be prepared by heating NaF and MnO2 with conc. H2SO4. Why?
The reason is that fluorine is the most electronegative element. Thus, MnO2 and H2SO4 are not able to oxidise F- ion to F2.



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