First class, and you are already being asked to do something a chemist spent a career on. On the bench in front of you is a deck of cards. Each card carries the measured properties of one element and a code letter instead of a name. Nothing tells you which element it is, and the periodic table is face down on the side table until the end of the period.
Sort them.
What you are trying to find out
The periodic table you have seen since Grade 9 is presented as a fact about nature. It is really an argument — somebody looked at a pile of measurements, decided which ones mattered, and proposed an arrangement. The arrangement survived because it kept being right about elements nobody had found yet.
Today you get the pile of measurements without the answer. The question is whether the arrangement is forced by the data, or whether a reasonable person could have come out somewhere else.
Two sub-questions, and you will answer both:
- Do the cards fall into families — sets that resemble each other more than they resemble anything else?
- If you line the cards up in order of increasing atomic mass, does anything come back round? A property that rises, falls, and rises again is doing something a list of unrelated substances would not do.
What you have to work with
- A deck of about twenty element cards, coded by letter. Each card gives: atomic mass, atomic radius in picometres, first ionisation energy in kilojoules per mole, electronegativity, melting point in degrees Celsius, density, whether the solid conducts electricity, and a one-line note on how the element behaves with air, with water, and with dilute acid.
- Large sheets of paper, and something to move the cards around on.
- A spot plate, dilute hydrochloric acid at 1.0 mol/L, and small samples of four metals — magnesium ribbon, zinc, iron, and copper — for the bench check in the second half.
- A stopwatch or a phone timer, and the balance.
Your design decisions, written down before you start:
- Which property you sort on first. You cannot use all eight at once. Pick one, sort by it, and see whether the groups it produces agree with the groups a different property produces. Two properties that give the same grouping are telling you something; two that disagree are telling you something more interesting.
- What counts as “the same family”. Write the rule down. “These three feel similar” is not a rule. “First ionisation energy below 600 kJ/mol and reacts with cold water” is a rule, and it can be wrong.
- Whether any card refuses to fit, and what you do with it. Do not quietly file the awkward one. Put it to one side and label the pile does not fit my rule.
- How long you will watch each metal in acid before you write “no reaction”, and what resolution your timer actually has.
Dilute acid, four metals, and hydrogen gas
This is a small procedure with one real hazard in it.
- The acid is 1.0 mol/L hydrochloric acid — the school dilution, not the stock bottle. It will not burn you instantly and it will damage an eye. Eye protection stays on.
- Use two or three drops in each well of the spot plate. There is no version of this where more acid gives you a better answer, and a full well is a well that slops onto your hand.
- The metals release hydrogen gas, which burns. There are no flames, no burners, and no hot plates anywhere in the room during this part. Not on the far bench either.
- Magnesium reacts fast. Use a piece about a centimetre long. It will fizz hard and the well will warm noticeably.
- Keep the spot plate flat on a tray and do not carry it across the room while it has acid in it.
- Acid on skin: rinse under running water for several minutes and tell me. Acid in an eye: eyewash immediately, and somebody else comes and gets me — you do not walk anywhere.
- Leftover acid goes in the labelled waste beaker. Nothing goes back into the stock bottle, and the metals go in the solids waste, not the sink.
The prediction you write first
In your journal, before you touch a card:
- How many families you expect to find, and roughly how big.
- Which property you think will sort most cleanly, and why you think so.
- For the bench check: rank the four metals — magnesium, zinc, iron, copper — from most reactive with dilute acid to least, and give a reason for the ranking that is not “I remember this”.
Write a number wherever a number is possible. “Magnesium will fizz within 5 s and copper will show nothing in 300 s” is a prediction that can fail. “Magnesium will react more” is not.
What to collect
Your sorting record. For each family you propose, one row:
| Family label | Cards in it | The rule that put them there | Cards that nearly fit, and why you excluded them |
|---|---|---|---|
Your periodicity check. Line the cards up by increasing atomic mass and plot one property against position. Atomic radius and first ionisation energy both work well.
| Position in the mass order | Card code | Property plotted | Value, with units |
|---|---|---|---|
The bench check. Record the resolution of your timer before the first reading.
| Metal | Time to first visible bubble (s) | Description at 30 s | Description at 300 s | Rank |
|---|---|---|---|---|
| Magnesium | ||||
| Zinc | ||||
| Iron | ||||
| Copper |
“No visible change in 300 s” is the honest entry for a metal that did nothing. It is not the same sentence as “no reaction”, and the difference matters at the end of this page.
What to bring to the consolidation discussion
- Your families, your rule for each, and your leftover pile.
- The plot, and one sentence saying whether the property came back round or climbed steadily.
- Whether your bench ranking matched your prediction, and where it did not.
- The gap question. If your ordering leaves a hole — a place where the pattern says there should be a card and there is not — say where it is and what properties the missing card would have to have. That question is the entire reason this table beat every rival arrangement.1
What you should not claim
- A sorting that works is not a sorting that is right. Twenty cards can be grouped several defensible ways. What raises one arrangement above another is whether it predicts something you did not use to build it — which is why the gap question above is worth more than a tidy set of piles.
- “No visible change in 300 s” is a statement about your eyes and your five minutes, not about the element. A reaction slower than your patience is invisible to this method. Iron and dilute acid at room temperature is exactly the case where this bites: whether you record it as reacting depends on how long you looked, and a group that waited ten minutes will rank it above a group that waited two.
- Your ranking of four metals is not an activity series. Four points is not an order, and one reagent is not a test. Where this goes next is Building the Activity Series, with a full grid.
- Warmth is not proof of a reaction, and bubbles are not proof of hydrogen. You have identified neither the gas nor the product today. You watched something happen and timed it.
- Surface condition is a real confounder, and it pushes one way. Metals in a drawer grow a thin oxide layer. That layer delays the start of the reaction, so an old, dull piece looks less reactive than a freshly cut one of the same element. If your zinc looked sluggish, ask whether you measured the zinc or measured the oxide.
Curriculum connection
B2.2
analyse data related to the properties of elements within a period (e.g., ionization energy, atomic radius) to identify general trends in the periodic table [AI]
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B2.3
use an inquiry process to investigate the chemical reactions of elements (e.g., metals, non-metals) with other substances (e.g., oxygen, acids, water), and produce an activity series using the resulting data [PR, AI]
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A1.1
formulate relevant scientific questions about observed relationships, ideas, problems, or issues, make informed predictions, and/or formulate educated hypotheses to focus inquiries or research
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Footnotes
-
Dmitri Mendeleev published an arrangement in 1869 that left empty spaces rather than closing up the ranks, and he stated what the missing elements would be like — their approximate masses, their densities, the formulas of their oxides. Elements matching those descriptions were isolated over the following two decades, and the predictions turned out to be close. Other chemists had noticed periodicity too; what made this version win was that it stuck its neck out and could have been shown wrong. ↩