Reactivity is ranked from evidence: how vigorously an element reacts, or whether it can displace another. A pattern such as “reactivity increases down Group I” is a summary of that evidence, not the starting point.
These questions appear as results tables followed by “put the elements in order” or “predict what happens with…”. The habit here is to read the table before reaching for memory.
What does the evidence look like?
Two kinds of evidence cover most questions in periodic patterns.
Reaction with water (Group I). Teachers demonstrate these reactions behind a safety screen, and you interpret the observations. The stronger the reaction, the higher the reactivity. Typical observation sets, in order of increasing vigour, run from steady fizzing to melting into a ball that moves across the surface to a reaction that ignites the gas.
Displacement (Group VII). A halogen mixed with a solution of a halide either produces a new colour (a reaction) or does not. A reaction means the added halogen is more reactive than the halogen already in the compound.
Worked example
The table is invented data for practice. A tick means a colour change showing displacement occurred; a cross means no change.
| Halogen added | With potassium chloride | With potassium bromide | With potassium iodide |
|---|---|---|---|
| Halogen A | cross | tick | tick |
| Halogen B | cross | cross | tick |
| Halogen C | cross | cross | cross |
| Halogen D | tick | tick | tick |
Put A, B, C and D in order of reactivity, most reactive first.
Step 1, count displacements for each: D three, A two, B one, C none.
Step 2, rank: more displacements means more reactive. The order is D > A > B > C.
Step 3, state it as a conclusion: “Halogen D displaces all three halides, so it is the most reactive. Halogen C displaces none, so it is the least reactive.”
Step 4, test with a pattern: if these were real halogens placed in Group VII from top to bottom, the most reactive is highest in the group. So D sits above A, then B, then C.
A real example of one row: Cl₂ + 2KBr → 2KCl + Br₂. Check the atoms: 2 Cl, 2 K, 2 Br on each side. Balanced.
What mistake goes wrong here?
The common slip is predicting from position by habit, using the wrong direction.
Mistaken answer: “Iodine is lower down the group, so it is more reactive and will displace bromine from potassium bromide.”
This borrows the Group I direction. For halogens, reactivity decreases going down, so iodine is less reactive than bromine and does not displace it.
The correction: decide the direction from the evidence. Chlorine displaces bromide and iodide, bromine displaces iodide only, and iodine displaces neither. Bromine + potassium iodide gives a reaction (Br₂ + 2KI → 2KBr + I₂); iodine + potassium bromide gives none.
Check yourself
1. Potassium bromide solution is added to bromine water and to iodine solution. Which mixture, if either, shows a reaction?
Show answer
Neither. Bromide ions are already bromine’s partner, so bromine has nothing to displace. Iodine is less reactive than bromine, so it cannot displace bromine either. No reaction in both.
2. In an invented data set, metal P floats and fizzes slowly, metal Q melts and moves quickly, and metal R ignites. Rank them from least to most reactive.
Show answer
More vigorous observations mean higher reactivity: P < Q < R.
3. Write the balanced equation for chlorine displacing iodide from potassium iodide.
Show answer
Cl₂ + 2KI → 2KCl + I₂. Atoms left: 2 Cl, 2 K, 2 I. Atoms right: 2 K, 2 Cl, 2 I. Balanced.
Where this leads next
The next lesson, explaining a trend without treating all groups alike, gives the reason behind the two directions you just used. To check your equations, use the equation balance reasoning trainer, and for amounts in reactions see the mole and equation-ratio tutor.
A teacher in online one-to-one Chemistry tuition can give you fresh results tables until reading evidence becomes routine.