The Argon Radius That Breaks the Period 3 Trend
In a Kerala school laboratory, a student records the atomic radii (in picometres) of the Period 3 elements from a data booklet: Na 186, Mg 160, Al 143, Si 118, P 110, S 104, Cl 99 and Ar 192. She notices that the radius decreases from Na to Cl as expected, but Ar is larger than Cl. She concludes that the Ar value must be a printing mistake and writes ‘radius always decreases across a period’ in her record book. Her partner disagrees, pointing out that argon is used in discharge tubes and is chemically unreactive. Their teacher asks them to check what kind of radius is being quoted rather than simply rejecting the data. The same booklet lists the oxides Na2O, MgO, Al2O3, SiO2, P4O10, SO3 and Cl2O7.
1. Is the argon value a printing mistake? Justify your answer and state how the period trend should be described.
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No. The values for Na–Cl are radii of bonded atoms (metallic or covalent radii), whereas the quoted radius of Ar is a van der Waals radius measured between non-bonded argon atoms. These are different quantities, so comparing 192 pm directly with Cl's 99 pm is invalid. Across Period 3, nuclear charge rises by one per element while the added electrons enter the same third shell; effective nuclear charge increases, so the radius of bonded atoms contracts. Ar is monatomic and does not form normal covalent bonds in this context, so its value is an exception of measurement type, not a violation of the trend. The correct statement is: atomic radius decreases across the period for elements that form bonds; noble-gas radii are quoted separately and are not used to extend the trend.
2. Classify the oxides Na2O, MgO, Al2O3, SiO2, P4O10, SO3 and Cl2O7, and explain how the classification reflects the metallic-to-non-metallic change across Period 3.
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Na2O and MgO are basic; Al2O3 is amphoteric; SiO2 is weakly acidic; P4O10, SO3 and Cl2O7 are acidic. Reasoning: Na and Mg are metals, so their oxides react with water or acids to give alkaline or salt-forming behaviour (Na2O + H2O → 2NaOH; MgO + H2O → Mg(OH)2). Al is at the metal–non-metal border, so Al2O3 reacts with both acids and bases. Si, P, S and Cl are non-metals; their oxides are acidic and react with bases to form salts (e.g., SO3 + 2NaOH → Na2SO4 + H2O). The change from basic to acidic oxides follows the decrease in metallic character and increase in non-metallic character across the period. Cl2O7 is the most acidic oxide in the list, consistent with Cl being the most non-metallic element shown.
3. Which element in Period 3 would you expect to form an ion with a 2− charge, and how does its position support your answer?
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Sulfur. Its electronic configuration is 2,8,6; it needs two electrons to reach the stable argon configuration, so it forms S2−. Position: it lies to the right of Si and P and to the left of Cl, in the non-metal region where the tendency to gain electrons is high. The 2− charge is typical of Group 16; oxygen above it also forms O2−. This is not simply name recall: the 2− charge follows from the two-electron gap to the next noble-gas configuration, and the position explains why sulfur is non-metallic and forms acidic oxides.