Identifying an Unknown Period 3 Element from Successive Ionisation Energies
A laboratory receives a sealed ampoule containing a sample of a Period 3 element, but the label has been lost. The sample is a brittle, lustrous solid that conducts electricity poorly. Analysis shows its first four ionisation energies (in kJ/mol) are approximately 1012, 1907, 2914, and 4956, followed by a very large jump to about 6274 for the fifth. The element reacts with chlorine to give a chloride with the formula ECl3 that fumes in moist air. A technician suggests the element could be aluminium because aluminium also forms AlCl3 and is a metal. A second technician argues the ionisation data point elsewhere. The sample mass is 2.70 g and it was found to react completely with 0.30 mol of chlorine atoms in a separate controlled experiment.
1. Using the ionisation energies, deduce the group of the element and explain the reasoning. Then decide which technician is correct.
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The key is the size of the jump between successive ionisation energies. The first four values rise smoothly (1012 → 1907 → 2914 → 4956), but the fifth is dramatically higher (6274). A large jump occurs when an electron is removed from a new, inner shell that is closer to the nucleus and less shielded.
Here the big jump is between the 4th and 5th electrons, meaning the element has four valence electrons. That places it in Group 14 (IVA), not Group 13. Aluminium (Group 13) would show its large jump after the third electron, not the fourth. So the first technician's aluminium suggestion is wrong on the ionisation evidence.
Period 3 Group 14 element is silicon. Silicon is a metalloid: brittle, lustrous, a poor conductor - consistent with the physical description. The formula ECl3 is a red herring; silicon actually forms SiCl4, which fumes in moist air by hydrolysis. The ionisation pattern is the decisive evidence.
2. The reaction experiment gave 2.70 g of sample reacting with 0.30 mol of chlorine atoms. Test whether this is consistent with silicon, and comment on whether the stated chloride formula ECl3 can be correct.
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Molar mass of silicon is about 28.1 g/mol. Moles of sample = 2.70 / 28.1 ≈ 0.096 mol.
If the element formed ECl3, the chlorine-to-element ratio would be 3:1, requiring 3 × 0.096 ≈ 0.29 mol Cl. That is close to 0.30 mol, so the raw numbers superficially fit a 1:3 ratio.
However, we must check consistency with the ionisation evidence, which fixes the element as silicon in Group 14. Silicon forms SiCl4, a 1:4 ratio, requiring 4 × 0.096 ≈ 0.38 mol Cl - which does not match 0.30 mol.
This is a genuine conflict. The most defensible resolution: the ionisation data are far more reliable for group assignment than a single mass/mole measurement, which carries weighing and purity error. A likely explanation is that the sample was not pure silicon (e.g., partly oxidised), reducing the effective reactive mass. The stated ECl3 formula cannot be correct for a Group 14 element; the correct chloride is SiCl4. The candidate should flag that the mass data are inconsistent and require re-measurement.