Across a period, increasing proton count generally strengthens effective attraction within the same shell and shrinks radius. Down a group, an added shell and more shielding place valence electrons farther away. Ionization energy generally rises when an electron is harder to remove.

The model has a domain of validity. Explain its assumptions before using it in a changed context, then compare your prediction with evidence.

• Radius and attraction follow three levers: nuclear pull, distance from the nucleus and shielding by inner electrons.
• Trend arrows are general patterns, not exception-free laws. Explain the competing causes.