The big idea: Elements in the same group have the same number of outer (valence) electrons, so they react in the same way — but the vigour of that reaction changes as you go down the group.
It all comes down to how easily an atom gains or loses an outer electron. Going down a group adds more electron shells, so the outer electrons sit further from the nucleus and feel more shielding — they are held less tightly.
Group 1 (the alkali metals) is the far-left column. Reactivity INCREASES going down it.
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Define the terms: - Group — a vertical column; same number of outer electrons. - Reactivity — how readily and vigorously an element reacts. - Shielding — inner shells of electrons screening the outer electrons from the nuclear charge. - Atomic radius — the size of the atom (grows down a group).
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Group 1 (alkali metals) react by losing their one outer electron. Going down the group the outer electron is further out and more shielded, so it is lost more easily — reactivity increases down the group.
| Group 1 metal | Reaction with water | Trend |
|---|---|---|
| lithium (Li) | fizzes gently | ↑ least reactive (top) |
| sodium (Na) | fizzes fast, may melt into a ball | |
| potassium (K) | ignites with a lilac flame | ↓ most reactive (bottom) |
Group 1 — increases down: Down group 1: bigger atom → outer electron further from the nucleus → held less tightly → easier to lose → more reactive.
So Li < Na < K < Rb < Cs in reactivity.
Group 17 (halogens) react by gaining one electron. Going down the group the atom is bigger, so an incoming electron is added further out and is less strongly attracted — gaining an electron gets harder, so reactivity decreases down the group.
Group 17 (the halogens) is the second-from-right column. Reactivity DECREASES going down it.
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Group 17 — decreases down: Down group 17: bigger atom → an incoming electron is added further from the nucleus and feels more shielding → harder to gain → less reactive.
So F > Cl > Br > I in reactivity. (Fluorine is the most reactive halogen; iodine the least.)
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Moving across a period (left → right) the nuclear charge rises while electrons go into the same outer shell, so atoms hold their electrons more tightly. Elements change from metallic (lose electrons easily) on the left to non-metallic (gain/share electrons) on the right. So metallic character decreases across the period.
Across period 3 (left → right) elements change from metallic (Na, Mg, Al) to non-metallic (Si, P, S, Cl, Ar).
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The acid–base pattern of the oxides: This metal → non-metal change shows up clearly in the oxides across period 3:
- Metal oxides (Na2O, MgO) are basic. - Aluminium oxide (Al2O3) is amphoteric — it reacts with both acids and bases. - Non-metal oxides (P4O10, SO3) are acidic.
So the oxides go basic → amphoteric → acidic from left to right.
| Period-3 oxide | Element type | Acid–base nature | Reaction in water |
|---|---|---|---|
| Na2O, MgO | metal | basic | forms an alkaline solution (or none for MgO) |
| Al2O3 | metalloid-ish metal | amphoteric | reacts with both acids and bases |
| SiO2 | metalloid | acidic (weakly) | insoluble; reacts with strong base |
| P4O10, SO3 | non-metal | acidic | forms an acidic solution |
Define: amphoteric: Amphoteric means a substance can act as both an acid and a base — it reacts with acids and with alkalis. Al2O3 (and Al(OH)3) is the classic example, sitting right at the metal/non-metal boundary.
How this is tested: This micro shows up two ways:
- Paper 1A (MCQ): pick the most/least vigorous reaction for a group-1 + group-17 pair using both trends at once. - Paper 2: an explain mark — why a trend happens (in terms of atomic radius and shielding) — or predict/sketch how a group-1 metal reacts, or judge whether an oxide/hydroxide is amphoteric.
The markers want the reason (radius + shielding), not just the direction of the trend.
Score the explain mark: Always link the trend to a cause: bigger atomic radius + more shielding → outer electron held less tightly. Name the cause, then state the effect on losing/gaining an electron.
IB-style question — group 1 reactivity (a)
(a) Explain why potassium is more reactive than lithium with water. [2]
How to score the marks
- Mark 1 — the cause. Potassium is lower in group 1, so it has more electron shells: a larger atomic radius and more shielding of the outer electron from the nucleus.
- Mark 2 — the effect. The outer electron is therefore held less tightly and is lost more easily, so potassium reacts more vigorously (more reactive).
Final answer
K is lower down: larger atom + more shielding → outer electron held less tightly → lost more easily → more reactive.
IB-style question — period-3 oxides (b)
(b) Aluminium oxide, Al2O3, is amphoteric, whereas sulfur trioxide, SO3, is acidic. Explain this difference in terms of the position of the elements in period 3. [2]
How to score the marks
- Mark 1 — amphoteric Al2O3. Aluminium sits near the metal/non-metal boundary, so its oxide has both metallic and non-metallic character — it reacts with acids and bases (amphoteric).
- Mark 2 — acidic SO3. Sulfur is a non-metal on the right of period 3, so its oxide is acidic (forms an acidic solution / reacts with bases only).
Final answer
Al is at the metal/non-metal boundary → amphoteric oxide (reacts with acid and base); S is a non-metal → acidic oxide.