Classification of Elements and Periodicity in Properties

Chemistry · Class 11

Simulation · Chemistry · Class 11

Which atom welcomes an extra electron?

From the lesson Electron gain enthalpy and electronegativity in Classification of Elements and Periodicity in Properties. Change the values and watch what happens.

Which atom welcomes an extra electron?Chemistry · Class 11

The idea behind it

NCERT §3.7.1(d), (e)

  • Electron gain enthalpy ΔegH is the enthalpy change when a neutral gaseous atom takes an electron: X(g) + e⁻ → X⁻(g). It can be negative (energy released) or positive (energy needed).
  • Halogens have strongly negative ΔegH because one electron completes a noble gas configuration. Noble gases have large positive values because the electron must enter a new, higher shell. The most negative values are at the upper right, just before the noble gases.
  • Trend, less regular than ionization enthalpy: ΔegH usually becomes more negative across a period (smaller atom, higher effective nuclear charge) and less negative down a group (larger atom).
  • Exception: O and F are less negative than S and Cl. The electron entering the compact n = 2 shell is strongly repelled; in n = 3 there is more room. So Cl (−349) is more negative than F (−328), and of P, S, Cl and F, Cl is most negative and P least.
  • NCERT values (kJ mol⁻¹): H −73; Li −60, Na −53, K −48, Rb −47, Cs −46; O −141, S −200, Se −195, Te −190, Po −174; F −328, Cl −349, Br −325, I −295, At −270; He +48, Ne +116, Ar +96, Kr +96, Xe +77, Rn +68.
  • Many books use electron affinity Aₑ, the negative of ΔegH, so a release of energy gives a positive Aₑ. Defined at absolute zero, it relates as ΔegH = −Aₑ − (5/2)RT.
  • Electronegativity is a qualitative measure of how strongly an atom in a compound pulls shared electrons to itself. It cannot be measured directly and changes with the bonding partner. Scales: Pauling (most used), Mulliken-Jaffe, Allred-Rochow; Pauling (1922) fixed fluorine at 4.0.
  • Pauling values: Li 1.0, Be 1.5, B 2.0, C 2.5, N 3.0, O 3.5, F 4.0; Na 0.9, Mg 1.2, Al 1.5, Si 1.8, P 2.1, S 2.5, Cl 3.0; down group 1: Li 1.0, Na 0.9, K 0.8, Rb 0.8, Cs 0.7; down group 17: F 4.0, Cl 3.0, Br 2.8, I 2.5, At 2.2.
  • Electronegativity rises across a period and falls down a group, following the inverse of atomic radius, like ionization enthalpy. It goes with non-metallic character and against metallic character.