what is the trend in the atomic radii as the atomic number increases across a period? What is the exception to this trend?
I know the radius decreases as you go from left to right, but I can't figure the exception, I read somewhere that it's the noble gasses but I'm not sure.
The trend in atomic radii across a period is that the atomic radius generally decreases as the atomic number increases from left to right. This is primarily due to an increase in the effective nuclear charge, which attracts the electrons more strongly, causing the atomic size to decrease.
The exception to this trend is indeed the noble gases. Noble gases generally have larger atomic radii compared to the elements in the same period. This is because noble gases have a completely filled electron shell and exhibit a stable electron configuration. The strong repulsion between the electrons in the filled shell counteracts the increased effective nuclear charge, leading to larger atomic radii.
You're correct that the trend in atomic radii across a period is that it generally decreases as the atomic number increases from left to right. This trend occurs due to the increasing effective nuclear charge, which attracts the outermost electrons more strongly towards the nucleus, thereby reducing the atomic radius.
The exception to this trend is indeed the noble gases. The noble gases, which include helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), and radon (Rn), are elements located in Group 18 of the periodic table. Unlike other elements, noble gases have completely filled electron orbitals, which results in a stable electron configuration. Due to this stability, noble gases have larger atomic radii compared to other elements in the same period. This is because the additional shielding effect from the filled electron shells reduces the attractive force between the positively charged nucleus and the outermost electrons.
To conclude, while the atomic radii generally decrease across a period as the atomic number increases, the noble gases have larger atomic radii due to their stable electron configuration.