01Explain with charge and distance
Across a period: protons increase, shielding barely changes → stronger pull → smaller radius, higher ionization energy and electronegativity.
Down a group: a new shell is added → electrons are farther away → larger radius, lower ionization energy.
02Notebook box
Radius ↓ across, ↑ down. Ionization energy & electronegativity ↑ across, ↓ down. Cations are smaller than their atoms; anions are larger.
Worked example: Why is Mg's 3rd ionization energy huge? Removing a 3rd electron breaks into the n = 2 core, much closer to the nucleus.