Periodic Trends

The position of an element on the periodic table predicts several measurable properties that vary in regular ways. These periodic trends arise from two competing factors: the effective nuclear charge ($Z_{eff}$) felt by the outer electrons, and the distance of those electrons from the nucleus, together with the shielding provided by inner electrons. Understanding these two factors allows you to reason through nearly every trend rather than memorizing them.

Atomic radius measures the size of an atom. Across a period, atomic radius decreases from left to right. Even though electrons are being added, they enter the same energy level while protons are added to the nucleus, so $Z_{eff}$ increases and pulls the electron cloud inward. Down a group, atomic radius increases because each successive