Question

why is chlorine more electronegative than sulphur ?

Answer 1

Electronegativity is a chemical property that describes the ability of an atom to attract electrons (or electron density) towards itself in a covalent bond. 

Sulfur and chlorine are in the same period (row) of the periodic table.
The nucleus of chlorine is larger than that of sulfur. 
Because of the larger size, the nucleus exerts a stronger pull on electrons. This stronger pull translates into a higher electronegatively for atoms in the same row of the periodic table.

Answer 2

Answer:

Chlorine (Z = 17) is placed after sulphur (Z = 16) in the same period i.e. third period. The size of chlorine is smaller than that of sulphur and its atom needs only one electron to have noble gas electronic configuration while sulphur atom needs two electrons. Therefore, chlorine has greater attraction for electrons than sulphur. It is more electronegative than sulphur.