Explain why the electron affinities of the elements of Period 3 are higher than those of the corresponding elements of Period 2 of the periodic table
Because all corresponding elements of the 3 period have higher nucleus charge than the elements of the 2 period. It is also governed with a so-called screening electrons i.e. electrons of the levels before the corresponding. In the period the number of screening electrons is not growing whereas the charge of the nucleus is growing.