Select the single best answer Rb has a lower IE1 than Sr, but Sr has a lower IE2

Question

Select the single best answer Rb has a lower IE1 than Sr, but Sr has a lower IE2 than Rb because: IE1 decreases from left to right across a period and IE2 increases from left to right across a period The radius of Rb is smaller than that of Sr. IE1 increases from left to right across a period. However, because Rb has only one valence electron, IE2 for Rb involves the removal of a core electron - requiring significantly more energy The radius of Sr is larger than that of Rb IE1 increases from left to right across a period and IE2 decreases from left to right across a period.

Explanation / Answer

In general Ionization energy icreases across the period from left to right, due to decrease in size and increase in nuclear charge. but this is only applicable to I.E1.

For Rb electronic conig. is [Kr] 5s1.

after 1st ionization Rb+ electronic config - [Kr] 5s0 ; which is inert gas config.

Similarly for Sr electronic config- [Kr] 5s2

after 1st ionization - [Kr] 5s1

When comparing the second ionization energy I.E2, Sr is gaining most stable inert gas config. Where as Rb is losing the stable config. Hence higher energy is needed for Rb than Sr

Correct answer is 3rd option.

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