The atomic radius of elements exhibits a clear trend across the periodic table, which is essential for understanding their properties. As one moves from left to right across a period, the atomic radius generally decreases. This decrease occurs because, while the number of electron shells remains constant, the number of electrons in the outer shell increases. This increase in electrons leads to a stronger attraction to the nucleus, effectively pulling the electrons closer and resulting in a smaller atomic radius.

Conversely, as one moves up a group in the periodic table, the atomic radius also decreases. This is attributed to the reduction in the number of electron shells, which means that the outermost electrons are closer to the nucleus, further contributing to a smaller atomic radius.

Atomic radius is typically measured in picometers (pm). For example, hydrogen has an atomic radius of approximately 37 pm, and as one progresses to helium, there is a slight decrease in size. However, it is important to note that there are exceptions to this trend, particularly among transition metals and the heavier elements in the last row of the periodic table. These elements, often synthesized in laboratories, can be unstable, leading to less definitive measurements of their atomic radii.

In summary, the general trend is that the atomic radius decreases as one moves towards the top right corner of the periodic table. Understanding this trend is crucial for answering questions related to atomic size and its implications in chemical behavior.