Q.Why is there a large difference between the melting and boiling points of oxygen and sulfur ?

Group 16, the oxygen family (O, S, Se, Te, Po), has the general valence-shell configuration ns2np4 and is also called the chalcogens, since a large number of metal ores are oxides or sulfides. Oxygen is the most abundant element on Earth (about 21% of air by volume, 47% of the crust by mass); sulfur occurs mainly as sulfates (gypsum, Epsom salt, baryte) and sulfides (galena, zinc blende, copper pyrites); selenium and tellurium occur as metal selenides/tellurides; polonium is a radioactive decay product of thorium/uranium. Down the group, atomic and ionic radii increase, ionisation enthalpy decreases, and electronegativity decreases, all because each successive element adds a new outer shell. Electron gain enthalpy becomes less negative down the group overall, but oxygen is an exception, showing a LESS negative value than sulfur, because oxygen's small, compact 2p orbitals suffer strong electron-electron repulsion when accepting an extra electron. Physically, oxygen is a gas while the rest of the group are solids; oxygen and sulfur are nonmetals, selenium and tellurium are metalloids, and polonium is the group's one true metal. Every element of the group shows allotropy, and every element shows the characteristic oxidation states -2 and +2, with the heavier members (S, Se, Te, Po) additionally reaching +4 and +6.