How will the tendency to gain electrons change as we go from left to right across a period? Why?
As we move from left to right across a period in the periodic table, the tendency to gain electron increases.
On moving from left to right across a period in the periodic table, there is an increase in the nuclear charge of the elements. At the same time, the size of the elements decreases. As a result, the effective nuclear charge acting on the valence electrons increases. Hence, the tendency to gain electrons increases.