Detailed Breakdown: Periodic Table & Trends
Effective Nuclear Charge ($Z_{eff}$): This is the net positive charge experienced by valence electrons. As we move across a period, more protons are added to the nucleus while the number of inner-shell shielding electrons remains constant. This increases $Z_{eff}$, pulling electrons closer and reducing the atomic radius.
Atomic and Ionic Trends: Periodicity describes repeating patterns across the table. For example, atomic radius decreases across a period but increases down a group. Conversely, first ionization energy and electronegativity generally increase across a period as the nucleus exerts a stronger grip on the electrons.
Chemical Periodicity and Reactivity: The metallic character of elements decreases across a period. Group 1 elements (Alkali metals) become more reactive down the group as their valence electron is further from the nucleus, while Group 17 (Halogens) become less reactive down the group.