Q.36 The elements with atomic numbers 19, 37 and 55
(A) form cubic chloride salts with the coordination number of cation being 6
(B) form ionic fluorides with general formula MF
(C) have lowest density of solids in their respective periods
(D) have lowest ionization energy in their respective periods
Elements with atomic numbers 19 (K), 37 (Rb), and 55 (Cs) are alkali metals in Group 1 of the periodic table. These elements exhibit distinct trends in ionization energy, density, and compound structures relevant to competitive exams like CSIR NET Life Sciences. Option (D) is correct as they have the lowest ionization energy in their respective periods (4, 5, and 6).
Option Analysis
Identify the elements: Atomic number 19 is potassium (K, period 4), 37 is rubidium (Rb, period 5), and 55 is cesium (Cs, period 6). All belong to Group 1 (s-block), with ns¹ configuration, increasing atomic size down the group.
(A) Cubic chloride salts, coordination number 6: Incorrect. KCl and RbCl adopt NaCl-type (rock salt, face-centered cubic) structure with octahedral 6:6 coordination. CsCl has body-centered cubic structure with 8:8 coordination. No uniform cubic structure with CN=6 for all.
(B) Ionic fluorides MF: Correct in formula (KF, RbF, CsF exist as MF). However, structures vary: smaller LiF/NaF are rock salt (6:6); larger KF/RbF/CsF adopt different polymorphs, but option fits generally without structure contradiction. Not the best distinguishing feature.
(C) Lowest density in periods: Incorrect. K (0.89 g/cm³) is low in period 4 (vs Ca 1.54); Rb (1.53 g/cm³) exceeds Sr (2.64) in period 5; Cs (1.93 g/cm³) below Ba (3.62) but not lowest vs heavier period 6 metals. Densities increase down group irregularly.
(D) Lowest ionization energy in periods: Correct. Group 1 elements have one valence electron, farthest from nucleus, yielding lowest first IE per period: K (419 kJ/mol) < Ca; Rb (403 kJ/mol) < Sr; Cs (376 kJ/mol) < Ba. Trend decreases down group due to larger size/weaker shielding.
The elements with atomic numbers 19, 37, and 55—potassium (K), rubidium (Rb), and cesium (Cs)—are key Group 1 alkali metals showing periodic trends in ionization energy, density, and ionic compounds, vital for CSIR NET Life Sciences preparation.
Periodic Trends
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Atomic size increases down the group, reducing effective nuclear charge on valence electron.
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First ionization energies: K (419 kJ/mol), Rb (403 kJ/mol), Cs (376 kJ/mol)—lowest in periods 4, 5, 6 due to ns¹ configuration.
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Densities: K (0.89 g/cm³), Rb (1.53 g/cm³), Cs (1.93 g/cm³); not lowest in periods.
Chloride Salt Structures
| Compound | Structure Type | Cation CN | Notes |
|---|---|---|---|
| KCl | NaCl (rock salt, FCC) | 6 (octahedral) | Similar to NaCl |
| RbCl | NaCl (normal conditions) | 6 | CsCl-type at high T/P |
| CsCl | CsCl (BCC) | 8 | Due to large cation/anion ratio |
No uniform CN=6 cubic chlorides.
Fluorides & Applications
All form MF (e.g., KF, RbF, CsF) ionic fluorides, used in specialty glasses and catalysts. Structures shift from 6:6 to higher CN down group.
Exam Relevance
For CSIR NET, focus on Group 1 trends: lowest IE confirms option (D). Practice radius ratio for structures (r⁺/r⁻ >0.732 favors 8:8).
