Final answer:
As we move from left to right across a period, the additional protons in the nucleus increase the Zeff on the outer electrons with only slight increases in shielding, resulting in progressively smaller atomic radii.
Step-by-step explanation:
In going left to right across a period in the periodic table, electron shielding has a minimal effect on the effective nuclear charge (Zeff) acting on the outermost electron in an atom. As the atomic number (Z) increases by one with each element, the increase in shielding is slight because electrons in the same valence shell do not shield each other efficiently. The consequent rise in Zeff causes the outermost electrons to be pulled closer to the nucleus, leading to a decrease in the atomic size or covalent radii. The increased nuclear attraction due to higher Zeff while moving across a period results in atoms with more electrons having a smaller radius, underscoring the significance of the electron shielding effect in atomic structure.