Discuss and compare the trend in ionisation enthalpy of the elements of group1 with those of group17 elements.

Answers (1)

Ionisation Enthalpy is defined as the change in the molar enthalpy when an atom loses an electron.

Factors Affecting Ionisation enthalpy:

  • Nuclear charge: As nuclear charge increases the Ionisation Enthalpy increases.
  • Size of the atom: As nuclear charge increases the Ionisation Enthalpy becomes less.

When one moves from lithium to fluorine across the second period, successive electrons are added to orbitals in the same principal quantum number, and the shielding of the nuclear charge by the inner core of electrons does not increase very much. Thus, across a period, increasing nuclear charge outweighs the shielding. Consequently, the outermost electrons are held more and more tightly, and the ionisation enthalpy increases across a period.

 As one goes down a group, the outermost electron being increasingly farther from the nucleus, there is an increased shielding of the nuclear charge by the electrons in the inner levels. So ionisation enthalpy decreases down the group.

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