question_answer

                       Both Na and H occur in group 1 of the periodic table ; yet melting point of\[NaCl\] is \[800{}^\circ C\] while that of\[HCl\]  is\[-114{}^\circ C\]. Why?

Answer:

                Though Na and H are present in the group 1, they differ in their electro negativities. The electro negativity of H is \[2\cdot 1\] and that of Na is \[0\cdot 9\]. Due to large electro negativity difference between Na and\[Cl\,(3\cdot 0-0\cdot 9=2\cdot 1),\]\[NaCl\]  is ionic in nature and exists as a crystalline solid with very high melting point (800°C). On the other hand, \[HCl\] is polar covalent due to small electro negativity difference\[(3\cdot 0-2\cdot 1=0\cdot 9)\] and exists in the gaseous state at room temperature. In order to solidify it, a very low temperature \[(-114{}^\circ C)\] is needed.