Compare bond distances without hiding unmatched evidence

Start with a difference you can defend

A table lists the carbon–carbon distance in ethane as 1.536 Å and in ethene as 1.339 Å. Before explaining why, calculate what changed. Target minus reference means ethene minus ethane: 1.339 − 1.536 = −0.197 Å. One ångström, Å, is 100 picometres, pm, so the change is −19.7 pm. The minus sign means shorter. It does not mean a negative bond length.

Enter your own signed prediction in the numerical control and capture it with a note before correcting it. The comparison graph uses the same scale for both reference distances, and a separate signed scale for your prediction. If you enter +19.7, the magnitude matches but the direction is reversed. The percentage change uses the original ethane distance: −0.197/1.536 × 100, about −12.826%. Changing the denominator changes the question.

These values are selected entries from NIST CCCBDB’s ethane record and ethene record. They are historical compiled experimental-reference entries, not measurements performed in this browser. Their source is Herzberg’s 1966 compilation. The selected rows give neither a numeric uncertainty nor a temperature/pressure pair. Missing uncertainty is not zero uncertainty.

A matching atom pair is a useful start

The carbon nuclei are the same elements in both molecules, and the simple Lewis descriptions change from a single to a double carbon–carbon bond. The shorter selected distance is consistent with the introductory bond-order trend. But hydrogen count and the surrounding electronic structure also change. This is a comparison between different molecules, not a way to change only one knob inside a real molecule.

Write a small matched-variable ledger in your capture note: same, changed, not reported. Keep “consistent with a trend” separate from “isolates the numerical effect of bond order.” A reference comparison can teach something even when it cannot establish the strongest causal claim.

The earlier potential-curve lesson deliberately varied well depth and equilibrium position independently in an original teaching model. That experiment isolated programmed parameters. It was not a fitted explanation of these real molecular records. Do not take a shorter reference distance and assign it an invented dissociation energy.

Read the distance definition

Now compare ethene with ethyne’s selected CCCBDB distance, 1.203 Å. Its row explicitly says rₘ structure, with Kuchitsu’s 1998 compilation as the source. The selected ethane and ethene rows do not specify rₑ or a particular averaging convention. We preserve that absence.

An equilibrium distance, rₑ, refers to a minimum-distance definition for the electronic potential. Effective or averaged structural parameters inferred from molecular motion need not be numerically identical to that minimum. The precise definition of an rₘ structural model needs its source methodology; this lesson does not silently turn it into rₑ. The ethene-to-ethyne difference is still calculable, −13.6 pm, but a calculation does not remove definition mismatch. There is no supplied uncertainty with which to test statistical significance.

Change the comparison question

The selected ground-state H³⁵Cl and H⁸¹Br distances are 1.274552 and 1.414435 Å. The second is 13.9883 pm longer. Here the simple Lewis bond order stays single while the halogen element changes. That is consistent with the role of atomic core size, but the electronic potential also changes; it is not an isotope-only control.

Finally select uncorrected versus corrected HCl. NIST note 31 explicitly distinguishes the tabulated value derived from Bₑ from a Born–Oppenheimer minimum of 1.2746149 Å. Their difference is 0.00629 pm, even though both round to the same 0.1 pm prediction. This is a source-definition/correction comparison for the same species, not evidence that a sample physically changed. The rounding feedback checks arithmetic only; it does not measure uncertainty or assess your explanation.

Transfer the reasoning

For each comparison, state the numerical result, then state the strongest supported interpretation and one missing observation. A new molecule pair can preserve the bonded elements while changing its environment or measurement definition. A useful next source would report compatible distance definitions, uncertainties and conditions. Continue to the isotope investigation to test another common shortcut: treating a lower vibration coefficient as direct evidence of a much weaker bond.