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NEET 2008
Exam
NEET
Year
2008
Subject
Chemistry
Chapter
p Block Elements (Group 15, 16, 17 & 18)

NEET 2008 · Chemistry · p Block Elements (Group 15, 16, 17 & 18)

The alkali metals form salt-like hydrides by the direct synthesis...

The alkali metals form salt-like hydrides by the direct synthesis at elevated temperature. The thermal stability of these hydrides decreases in which of the following orders?

Options

  1. ACsH>RbH>KH>NaH>LiH\mathrm{CsH}>\mathrm{RbH}>\mathrm{KH}>\mathrm{NaH}>\mathrm{LiH}
  2. BKH>NaH>LiH>CsH>RbH\mathrm{KH}>\mathrm{NaH}>\mathrm{LiH}>\mathrm{CsH}>\mathrm{RbH}
  3. CNaH>LiH>KH>RbH>CsH\mathrm{NaH}>\mathrm{LiH}>\mathrm{KH}>\mathrm{RbH}>\mathrm{CsH}
  4. DLiH>NaH>KH>RbH>CsH\mathrm{LiH}>\mathrm{NaH}>\mathrm{KH}>\mathrm{RbH}>\mathrm{CsH}

Correct answer

D. LiH>NaH>KH>RbH>CsH\mathrm{LiH}>\mathrm{NaH}>\mathrm{KH}>\mathrm{RbH}>\mathrm{CsH}

Step-by-step solution

Key Idea : Small anion forms stable compounds with small cation. The thermal stability of alkali metal hydrides decreases as : LiH\mathrm{LiH}>NaH\mathrm{NaH}>KH\mathrm{KH}>RbH\mathrm{RbH}>CsH\mathrm{CsH}because the size of cation increases as : Li+\mathrm{Li}^{+}< Na+\mathrm{Na}^{+}< \mathrm{K}^{+} < Rb+\mathrm{Rb}^{+}< Cs+\mathrm{Cs}^{+}
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