In a period, the first ionisation enthalpy of the element at extreme left and the negative electron gain enthalpy of the extreme right element, except noble gases, are respectively.
- lowest and lowest
- highest and lowest
- lowest and highest
- highest and highest
Answer
(3) lowest and highest
Solution
Extreme left of a period: alkali metal (one electron outside a noble-gas core, largest atom of the period)
⇒ lowest first ionisation enthalpy in the period
Extreme right (except noble gas): halogen (needs one electron to complete the octet, small size, high Zeff)
⇒ most negative electron gain enthalpy, i.e. the highest negative electron gain enthalpy
∴ lowest and highest
Answer: (3)