In general, as you move across a period from left to right on the periodic table, it is increasingly difficult to remove an electron from an atom. The ionization energy (energy required to do this) increases.
Remember that metals tend to lose electrons to form ions, while nonmetals tend to gain electrons to form ions, so electrons are easier to remove from metals than they are from nonmetals.
Ionization energy increases as you go from the bottom to the top of a group (or family) of elements. The smallest atomic radii are located at the top of a group, and the electrons are closer to the nucleus. The positive nucleus exerts more force on those electrons so it is harder to remove them.
