Lesson 7: 1.7 Periodic Trends

Why does the periodic table have the specific shape that it does?
How can Coulomb’s law, shielding, and effective nuclear charge (Zeff) be used to explain patterns across a period or down a group?

Periodicity
Effective Nuclear Charge (Zeff)
Shielding Effect
Atomic Radius
Ionic Radius
Ionization Energy
Electronegativity

Learning Objective 1.7.A; Suggested Skill 4.A (Model Analysis / Prediction)

This is historically a highly tested and structurally challenging topic for students. Students must move away from memorizing trends (e.g., "atomic radius increases down and left") and instead learn to write justifications using effective nuclear charge (Zeff) for rows and electron shielding/shells for columns. Properties include ionization energy, atomic/ionic radii, electron affinity, and electronegativity. Note: Exceptions to the Aufbau principle are excluded.

Support: Utilize a structured claim-evidence-reasoning (CER) writing frame to guide students through explanations, ensuring they always mention both nuclear charge and shielding.
Extension: POGIL activity: Provide ionization energy data exhibiting anomalies (e.g., the drop in ionization energy between Nitrogen and Oxygen) and have students rationalize the exception using electron-electron repulsions in orbital diagrams.

An AP-style free-response question requiring students to compare the atomic radius or ionization energy of two elements and justify the difference using principles of atomic structure (not just location on the table).