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9476 / 2027
The Periodic Table

Lesson 6 of 6

Predict and identify

Combine independent physical and chemical clues.

A-Level 9476 (2026-2027)

  • I can predict properties from group membership.Syllabus

    Syllabus 9476, 5(h). Predict properties from group membership.

  • I can infer periodic position and identity from evidence.Syllabus

    Syllabus 9476, 5(i). Infer periodic position and identity from evidence.

Combine the clues before assigning an element

An inference is strongest when configuration, physical properties and reactions agree.

Worked exampleIdentify a period-3 element

A period-3 element conducts electricity. Its hydroxide precipitate dissolves in both acid and excess aqueous NaOH. Its chloride gives an acidic solution. Which element is most consistent?

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  1. Electrical conduction favours one of the metals, rather than a molecular non-metal.
  2. Amphoteric hydroxide behaviour identifies aluminium among the period-3 metals.
  3. Acidity from hydrolysis of hydrated Al3+ is consistent with the chloride clue.
Answer

Aluminium. The given period matters: amphoteric behaviour alone would not uniquely identify an element across the entire table.

Worked examplePredict a Group-2 comparison

How should strontium compare with magnesium in ionic radius, first ionisation energy and carbonate thermal stability?

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  1. Sr2+ has more occupied shells, so its ionic radius is larger.
  2. Greater shielding and distance generally lower the first ionisation energy of Sr.
  3. The larger Sr2+ ion has lower charge density and polarises carbonate less strongly.
Answer

Larger ion, lower first ionisation energy, more thermally stable carbonate. Each prediction uses the mechanism appropriate to that property.

When practical evidence is supplied, distinguish observation from interpretation. "A white precipitate dissolves in excess alkali" is an observation; "the hydroxide is amphoteric" is an interpretation. Use the stated temperature, physical state and reagent excess. If two identities fit, name the additional evidence needed instead of forcing a unique answer.

Check your understanding

A Period 3 element forms a chloride that is fully hydrolysed in water to give a solution of pH about 1-2 and an acid containing the element. Which element fits?

  1. Aluminium, as AlCl3 gives about pH 3 in water
  2. Sodium, as NaCl dissolves to give ions in water
  3. Phosphorus, as PCl5 hydrolyses to H3PO4 and HCl
Show the answer

Phosphorus, as PCl5 hydrolyses to H3PO4 and HCl

PCl5 + 4H2O -> H3PO4 + 5HCl. Both acids dissolve, giving a strongly acidic solution.