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Periodic Table and Periodicity question

2020 · 7 Jan · Shift 1 · Q15
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Periodic Table and Periodicity question

2020 · 7 Jan · Shift 1 · Q15

JEE MainChemistryPeriodic Table and PeriodicityMCQ+4 / −1
The electron gain enthalpy (in KJ/mol) of fluorine, chlorine, bromine and iodine, respectively are:
  1. A
    -296, -325, -333 and -349
  2. B
    349, -333, -325 and -296
  3. C
    -333, -349, -325 and -296
  4. D
    -333, -325, -349 and -296
View written solutionFree

Correct answer: C

  1. Recall the trend of electron gain enthalpy in halogens

Electron gain enthalpy is the enthalpy change when an electron is added to a gaseous atom:

X(g)+e−→X−(g)X(g) + e^- \to X^-(g)X(g)+e−→X−(g)

More negative value means greater tendency to gain an electron.

For halogens, electron gain enthalpy is generally highly negative, but there is an important exception:

  • Chlorine has more negative electron gain enthalpy than fluorine.
  • This is because fluorine is very small, so the incoming electron enters a compact 2p2p2p orbital and experiences greater electron-electron repulsion.

Thus, the order is:

Cl<F<Br<I(in terms of becoming less negative? No, let us write correctly by actual values)\text{Cl} < \text{F} < \text{Br} < \text{I} \quad \text{(in terms of becoming less negative? No, let us write correctly by actual values)}Cl<F<Br<I(in terms of becoming less negative? No, let us write correctly by actual values)

Actual approximate electron gain enthalpy values are:

  • Fluorine: −333 kJ mol−1-333\,\text{kJ mol}^{-1}−333kJ mol−1
  • Chlorine: −349 kJ mol−1-349\,\text{kJ mol}^{-1}−349kJ mol−1
  • Bromine: −325 kJ mol−1-325\,\text{kJ mol}^{-1}−325kJ mol−1
  • Iodine: −296 kJ mol−1-296\,\text{kJ mol}^{-1}−296kJ mol−1
  1. Match with the options

We need the sequence for fluorine, chlorine, bromine and iodine respectively:

(−333, −349, −325, −296)(-333,\,-349,\,-325,\,-296)(−333,−349,−325,−296)

Checking options:

  • A: −296,−325,−333,−349-296, -325, -333, -349−296,−325,−333,−349 ❌
  • B: 349,−333,−325,−296349, -333, -325, -296349,−333,−325,−296 ❌
  • C: −333,−349,−325,−296-333, -349, -325, -296−333,−349,−325,−296 ✅
  • D: −333,−325,−349,−296-333, -325, -349, -296−333,−325,−349,−296 ❌
  1. Final answer

Therefore, the correct option is:

C\boxed{\text{C}}C​

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