Which of the following pairs of elements would you expect to have lower first ionisation enthalpy? Explain your answer:
(a) Cl or F
(b) Cl or S
(c) K or Ar
(d) Kr or Xe.
(a) Cl or F: Chlorine is expected to have lower IE1 as compared to fluorine since ionisation enthalpy decreases down a group. Chlorine comes after fluorine in group 17 of the periodic table.
(b) Cl or S: Sulphur is expected to have lower IE1 as compared to chlorine since ionisation enthalpy increases along a period. Chlorine having smaller size than sulphur comes after sulphur in the third period.
(c) K or Ar: Potassium is expected to have lesser IE1 than argon (noble gas-element) because the configuration of argon is very much symmetrical stable. Hence argon has a very high IE, value.
(d) Kr or Xe: Both the elements belong to group 18 (noble gas family). As we know ionisation enthalpy decreases down the group, thus xenon comes after krypton has lesser IE1 value.
