The question is incomplete, the complete question is ;
Which of the following is the correct rank of increasing electron affinities (from less exothermic to more exothermic)? Select the correct answer below: O Cs <K<Na <Li O Li< Na <K <Cs O Na <Li<K <Cs OK<Cs < Li< Na
Answer:
Cs< K<Na<Li
Explanation:
The electron affinity is defined as the energy released when a neutral gaseous atom accepts an electron to create a gaseous negative ion. This reaction is exothermic and leads to the evolution of heat.
When an electron is added to a metal, large energy is required since metals are usually electro positive. Electron affinity decreases down the group since electrons enter into higher energy levels further away from the nucleus. They experience lesser nuclear attraction.
This is clearly evidenced in the sharp decrease in the electron affinity of group one elements from lithium to caesium. The electron affinity of lithium is 60Kjmol-1 but decreases steadily down the group to a value of 46KJmol-1 for caesium. This explains the order chosen in the answer.
