In any atom with more than two electrons, inner electrons have a repelling effect on outer electrons, reducing the net attraction of the nucleus for the outer electrons. This effect is known as shielding, and it is responsible for many chemical changes in elements near the bottom of the periodic table. Attraction between electrons and protons decreases with distance, and shielding further decreases it but does not remove all attraction.
The shielding effect of inner electrons on outer valence electrons combined with the positive charge of the nucleus results in what is known as the effective nuclear charge. This is expressed as a number equal to the number of protons in the atom minus the number of electrons beneath the outermost valence shell. Thus, while atomic radii tend to get larger as one reads the periodic table from top to bottom, they tend to get smaller as one reads it from left to right.
The shielding effect is the major reason why it tends to be easier to remove electrons from heavier atoms. It is extremely difficult to make the noble gases, such as helium and neon, react with other chemicals. In contrast, heavier gases, such as krypton and xenon, react with very electronegative elements under the right conditions.