Patterns across the table

Across many main-group periods, atomic radius tends to decrease while first ionisation energy tends to increase. Effective nuclear charge helps explain these trends.

Down a group, additional electron shells generally increase atomic size. Shielding affects how strongly outer electrons are attracted.

Treat trends as a way to ask better questions. Orbital filling and electron interactions produce exceptions. Compare actual values and keep the measurement definition consistent.

Practice: make a prediction, then explain it

Compare lithium and sodium: sodium is generally larger because its occupied outer shell is farther from the nucleus. Across a period, increasing nuclear attraction usually makes atomic radii smaller. These are trends with exceptions, not formulas that predict every measured value

Check your understanding

Does “farther right” always mean a higher first ionization energy?

Try it in the periodic table

Scientific reference: OpenStax Chemistry 2e

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