Question

How can electronegativity change themselves when in the Periodic System moving vertically down through a main group (for a higher atomic masses) and horizontally in the system (for a higher group numbers)

Answer 1

Electronegativity is increases in a group from top to bottom vertically because the atomic size of the elements is increases for higher atomic masses .

for example, 17th group elements, F - 4.0                       

                                                           Cl - 3.0

                                                           Br - 2.8

                                                           I - 2.5

Electronegativity is decreases in a period from left to right hoizontally because the atomic size of the elements is decreases for higher atomic masses.

for example 2nd period elements,  

Li-1.0, Be-1.5, B-2.0, C-2.5, N-3.0, O-3.5, F-4.0.