The trend in atomic radii across a period is that they generally decrease as the atomic number increases from left to right. This is because, as you move across a period, the number of protons in the nucleus increases, resulting in a stronger attractive force pulling the electrons closer to the nucleus.
The exception to this trend is the group of noble gases. Noble gases are found in Group 18 (VIII A) of the periodic table, and they include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn). The atomic radii of noble gases do not follow the regular trend across a period. Instead, they have larger atomic radii compared to the elements in the same period.
This exception occurs because noble gases have completely filled outer electron shells. The filled electron shells create a repulsive force between the electrons, which counteracts the attractive force from the nucleus. As a result, the outermost electrons in noble gases are more shielded from the attractive pull of the nucleus, creating larger atomic radii compared to other elements in the same period.