Final answer:
Atomic radii decrease across a period due to increased nuclear charge tightening the electron cloud and increase down a group due to additional electron shells that cause increased electron shielding.
Step-by-step explanation:
The variations in atomic radii across the periodic table are influenced by two main factors: nuclear charge and electron shielding. The atomic radius tends to decrease across a period, and this is attributed to an increase in nuclear charge which leads to a stronger attraction of the electrons to the nucleus, thus pulling the electron cloud closer and reducing the radius. This can be summarized as follows: as the right arrow PT, the atomic radius decreases. Conversely, the atomic radius tends to increase down a group mostly due to increased electron shielding. When we move down a group, additional electron shells are added, resulting in larger atomic radii despite the increased nuclear charge. This can be represented as down arrow PT, atomic radius increases. It is important to note that variations in atomic radii are crucial for understanding the chemical and physical properties of the elements.