Atomic radius decreases across a period because the number of protons in the nucleus increases while the number of electron shells remains the same. As you move from left to right across a period, each new element has one more proton, one more electron and the same number of occupied energy levels. This additional proton increases the effective nuclear charge, the net pull that the nucleus exerts on the electrons. Since electrons are added to the same energy level and not to a new shell, they are drawn closer toward the nucleus. As a result, the entire electron cloud becomes more compact, causing the atomic radius to shrink.
Another key factor is electron shielding, which refers to how inner electrons block the attraction between the nucleus and outer electrons. Across a period, shielding does not significantly increase because no new inner shells are added. Instead, electrons are simply being added to the existing outer shell. This means the increase in nuclear charge is not counterbalanced by an increase in shielding. With the nucleus pulling harder while shielding remains nearly constant, the electrons experience a stronger attraction and move closer to the center.
A third reason for the decrease in atomic radius is the increasing effective nuclear charge (Zeff) acting on the valence electrons. Zeff increases steadily across a period, making it energetically favorable for the atom to hold its electrons more tightly. As Zeff rises, the atom contracts. This effect is strong enough to override the slight electron–electron repulsion that also grows as more electrons are added to the shell.
This trend continues until noble gases, which represent the natural limit for how tightly electrons can be pulled in while maintaining stable full-shell configurations. Although noble gases have larger electron–electron repulsions due to full shells, the strong nuclear attraction still dominates, keeping the radius small.
Overall, the decrease in atomic radius across a period reflects the balance between increasing nuclear charge, relatively constant shielding and stronger attraction acting on electrons in the same shell. These combined effects compress the electron cloud, creating a predictable periodic trend.
