Question

What element could I be talking about: It has more electrons than Rb but less than Cs. The last electron that fills its valence shell has a quantum number where l is 1 greater than ml but l is 4 less than n. (It is assumed that this element is neutral and all electrons are in their ground state.) (PS: There is more than one possibility!)

Answer 1

Rb has 37 electrons and Cs has 55 , so the element is having atomic number between 38 to 54.

l cannot be 0 , because l and ml are 0 for s.

The elements in this range have value of n as 4 and 5 , so l will be either 0 or 1.

for 0 it will be 4s or 5s and for 1 it will be 5p.

s is already ruled out , so we are now left with only 5p

There are 6 elements in this range.

Now since here l is 1 , and it is greater than ml by1 , so the ml is 0

So two elements fit this quantum number set , these are tin and iodine.