Why does electronegativity increase across a period?
The electronegativity is the tendency of an atom or a functional group to attract electrons (or electron density) towards itself.


This is because the attraction of bonding electrons by an atom increases with nuclear charge (Atomic Number) and decrease of atomic radius. Both these factors operate as we move to the right in period.


  Does it have something to do with the shielding effect of added electrons?


The electron cloud in the inner orbits act as a shield and reduces the nuclear attraction to the outer orbits. Because of the shielding effect, the tendency of nuclear attraction reduces and thus electronegativity reduces.