Final answer:
Atomic size decreases with increasing effective nuclear charge (Zeff). The trend occurs as additional protons in the nucleus exert a greater pull on the valence electrons, reducing atomic size across periods, while within groups, atomic size increases due to additional electron shells.
Step-by-step explanation:
Atomic size generally decreases with increasing Zeff (effective nuclear charge). The concept of effective nuclear charge is essential to understanding this trend. Effective nuclear charge refers to the net positive charge experienced by an electron in a multi-electron atom, and it is calculated as the nuclear charge (Z) minus the shielding effect caused by the presence of other electrons. As we move across a period from left to right, the number of protons in the nucleus increases, thus increasing the Zeff. This greater Zeff pulls the outermost electrons closer to the nucleus, leading to a smaller atomic radius.
Group Trend in Atomic Size
Contrarily, within a group, as the atomic number increases, so does the atomic size. This is because new electron shells are being added, which are farther from the nucleus and therefore less affected by the increased nuclear charge.