Chemistry · Classification of elements and periodicity · chapter 3 of 9

Class 11 Chemistry Chapter 3: Classification of Elements and Periodicity in Properties case study question with answers

One case-based question of the kind CBSE-pattern school exams ask: a passage and a table, then 3 parts worth 4 marks in all, with the answer and the marking scheme for each. Written from the NCERT chapter.

Case study · 4 marks

Aditi, a Class 11 student in Pune, makes a chart of the third-period elements for her chemistry project. For each element she writes its atomic radius and its electronegativity on the Pauling scale, taken from her textbook. She notices that as one quantity falls the other rises, and her teacher asks her to use the chart to predict the kind of bond two elements would form.

Element	Na	Mg	Al	Si	P	S	Cl
Atomic radius (pm)	186	160	143	117	110	104	99
Electronegativity	0.9	1.2	1.5	1.8	2.1	2.5	3.0

Part 1 · 1 mark

The element in the chart with the greatest metallic character is

(a) chlorine(b) aluminium(c) silicon(d) sodium

Show answer

Answer: (d) sodium

Metallic character is highest at the extreme left of a period, where the atom is largest and the electronegativity lowest: sodium (186 pm, 0.9).

Part 2 · 1 mark

Of the following pairs, the bond between which two elements would be the most ionic?

(a) Na and Cl(b) Mg and S(c) Al and Cl(d) Si and Cl

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Answer: (a) Na and Cl

The larger the difference in electronegativity, the more ionic the bond. Na–Cl: 3.0 − 0.9 = 2.1, the largest of the four (Mg–S 1.3, Al–Cl 1.5, Si–Cl 1.2).

Part 3 · 2 marks

Explain why the atomic radius falls and the electronegativity rises from sodium to chlorine.

Answer and marking scheme

Across the period the electrons enter the same shell (n = 3) while the nuclear charge increases from 11 to 17; shielding by the inner core hardly changes, so the effective nuclear charge rises and the electron cloud is drawn in: the radius falls from 186 to 99 pm. A smaller atom holds shared electrons closer to its nucleus and attracts them more strongly, so electronegativity rises from 0.9 to 3.0.

same shell, increasing effective nuclear charge: radius falls (1)
smaller atom attracts shared electrons more: electronegativity rises (1)

Using the group valences, write the formulas of the compounds formed by (i) magnesium with chlorine and (ii) phosphorus with hydrogen, and state which of the two is the more ionic.

Answer and marking scheme

Mg (group 2) has valence 2 and Cl (group 17) valence 1: MgCl₂. P (group 15) shows valence 3 with hydrogen: PH₃. MgCl₂ is the more ionic, since the electronegativity difference Mg–Cl (1.8) is far greater than in P–H, a bond between two non-metals of similar electronegativity; Mg is a metal at the left of the period.

MgCl₂ and PH₃ from valences (1)
MgCl₂ more ionic, from electronegativity difference / metal-non-metal (1)

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