Short on time? This page is the recap. For explanations, figures and worked examples, use the full study guide.
Recap
- Properties repeat (periodicity) because valence electron configurations repeat. Elements in a group have the same outer pattern, such as ns¹ for group 1.
- Each period fills a new outer shell and ends with a filled shell at a noble gas.
- Every trend is explained with Coulomb’s law (F ∝ q₁q₂ / r²), the shell model, shielding by core electrons and effective nuclear charge, Z_eff.
- Simple estimate: Z_eff ≈ protons − core electrons (Na +1, Cl +7, K +1).
Key relationships
| Property | Across a period → | Down a group ↓ | Reason |
|---|---|---|---|
| Atomic radius | decreases | increases | Z_eff rises in the same shell; a new shell is farther out |
| First ionization energy | increases (dips at groups 13 and 16) | decreases | stronger or weaker attraction for the outer electron |
| Electron affinity (energy released) | usually more | usually less | larger Z_eff or a closer shell attracts the added electron more |
| Electronegativity | increases | decreases | bonding pair held closer or by a larger Z_eff |
Ions: cations are smaller than their atoms (a shell is lost); anions are larger (extra repulsion, same protons). In an isoelectronic series, more protons means a smaller ion.
Successive ionization energies: each one is larger; the big jump shows that the next electron comes from an inner shell. Magnesium: 738, 1451, 7733 kJ mol⁻¹, so it forms Mg²⁺.
Assumptions behind the model
- Core electrons shield fully and valence electrons barely shield each other (the simple Z_eff estimate).
- An electron in a higher shell is, on average, farther from the nucleus.
- Trends are general patterns. Explain any exception from the configuration.
Mistakes to avoid
- “More shielding across a period.” Core shielding is about constant; nuclear charge changes.
- “More electrons, bigger atom.” Not across a period.
- “Half-filled is stable” as the whole explanation. Name the paired-electron repulsion.
- Describing instead of explaining. Give the Coulomb’s law reason, not just the arrow.
- Electron affinity vs electronegativity. Isolated atom gaining an electron vs atom attracting a shared pair in a bond.
- Sign confusion. Tables use different signs for electron affinity; compare how much energy is released.
Quick self-check
- Which is larger, Mg or Mg²⁺? (Mg: Mg²⁺ has lost its n = 3 shell.)
- Why is the first ionization energy of Al lower than that of Mg? (Al loses a 3p electron, higher in energy and more shielded than Mg’s 3s electron.)
- Put N³⁻, O²⁻ and F⁻ in order of increasing radius. (F⁻ < O²⁻ < N³⁻: all have 10 electrons, and fewer protons gives a larger ion.)
- Which is more electronegative, P or N? (N: its bonding electrons are in a shell closer to the nucleus.)
Next: practice questions.