Short on time? This page is the recap. For explanations, diagrams and worked examples, use the full study guide.
Recap
- Valence electrons are the outer-shell electrons beyond the noble-gas core. Main groups: groups 1–2 have 1–2; groups 13–18 have (group − 10).
- Whether two elements bond depends on how strongly each nucleus attracts valence electrons (Coulomb’s law: charge and distance). A weakly holding metal plus a strongly attracting nonmetal gives electron transfer and an ionic compound.
- Metals react by losing electrons, so they get more reactive down a group (valence electron further away, more shielded).
- Halogens react by gaining electrons, so they get less reactive down a group (incoming electron further from the nucleus).
- Elements in one group form analogous compounds: Na₂O, K₂O; MgCl₂, CaCl₂; H₂O, H₂S.
- Main-group ions take the electron arrangement of the nearest noble gas. The charge stops there because the next electron would come from (or go into) a different shell, which costs much more energy.
Key relationships
| Group | Valence e⁻ | Typical ion | Example compound |
|---|---|---|---|
| 1 | 1 | +1 | NaCl |
| 2 | 2 | +2 | MgO |
| 13 | 3 | +3 (Al) | Al₂O₃ |
| 15 | 5 | −3 | Ca₃N₂ |
| 16 | 6 | −2 | K₂S |
| 17 | 7 | −1 | CaF₂ |
| 18 | 8 (He 2) | none | — |
Ionic formula rule: smallest whole-number ratio of ions with total charge zero. Check: (number × charge) + (number × charge) = 0.
Assumptions behind the predictions
- Predictions from position apply to main-group elements. Transition metals often form more than one ion; use the charge given (iron(III) means Fe³⁺).
- “Typical” charge means the usual one in simple ionic compounds, not every compound the element can form.
Mistakes to avoid
- Subscripts are not charges. In Ca₃N₂, calcium is 2+.
- Reduce the ratio. MgO, not Mg₂O₂.
- Halogen trend runs the other way. Fluorine is the most reactive halogen.
- Nonmetal charges are negative. Sulfur forms S²⁻, not S⁶⁺.
- “Wants a full shell” is not an explanation. Use energy: a jump in successive ionization energy, or a new shell for an extra electron.
- Analogous ≠ identical. Same formula type; properties change gradually.
Quick self-check
- Formula of lithium nitride? (Li₃N)
- Formula of aluminium sulfide? (Al₂S₃)
- Successive ionization energies show the big jump after the second electron. Typical ion? (M²⁺)
- Which reacts more vigorously with water, Na or K? (K: its valence electron is further from the nucleus and more shielded)
Next: practice questions.