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Elements of Groups 16, 17 and 18 — Chemistry STD 12 Science — Question

Maharashtra BoardEnglish MediumSTD 12 ScienceChemistryElements of Groups 16, 17 and 184 Marks
Question
Explain the trend in the following properties of group $17$ elements.(1) Atomic size,
(2) Ionisation enthalpy,
(3) Electronegativity,
(4) Electron gain enthalpy.
✓

Answer

(1) Atomic size :
  • Atomic and ionic radii increase down the group as atomic number increases due to the addition of new electronic valence shell to each succeeding element.
  • The atomic radii increase in the order $F < Cl < Br < 1$
  • Halogens possess the smallest atomic and ionic radii in their respective periods since the effective nuclear charge experienced by valence electrons in halogen atoms is the highest.
(2) Ionisation enthalpy :
  • The ionisation enthalpies of halogens are very high due to their small size and large nuclear attraction.
  • The ionisation ethalpies decrease down the group since the atomic size increases.
  • The ionisation enthalpy decreases in the order $F > Cl > Br > I$.
  • Among halogens fluorine has the highest ionisation enthalpy due to its smallest size.
Element F Cl Br I
Ionisation enthalpy kJ/mol $1680$ $1256$ $1142$ $1008$
(3) Electronegativity :
  • Halogens have the highest values for electronegativity due to their small atomic radii and high effective nuclear charge.
  • Each halogen is the most electronegative element of its period.
  • Fluorine has the highest electronegativity as compared to any element in the periodic table.
  • The electronegativity decreases as,
    $F > Cl > Br > I$
    $4.0 3.2 3.0 2.7$ (electronegativity)
(4) Electron gain enthalpy $(\triangle _{eg}H)$ :
  • The halogens have the highest negative values for electron gain enthalpy.
  • Electron gain enthalpies of halogens are negative indicating release of energy.
  • Halogens liberate maximum heat by gain of electron as compared to other elements.
  • Since halogens have outer valence electronic configuration, $ns^2\ np^5$, they have strong tendency to accept an electron to complete an octet and acquire electronic configuration of the nearest inert elements.
  • In case of fluorine due to small size of 2 p-orbitals and high electron density, F has less negative electron gain enthalpy than $Cl$.
  • $F_{(g)} + e^– \rightarrow F^–_{(g)} \triangle _{eg}H = – 333 klmol^{-1}$
  •  ^$Cl_{(g)} + e^– \rightarrow Cl^–_{(g)} \triangle _{eg}H = – 349 kJ mol^{-1}$
  • The variation in electron gain enthalpy is in the order of, $Cl > F > Br > I$.

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