Question

Correct set of four quantum numbers for the valence (outermost) electron of rubidium ($5^{th}$ element in group I) is:

• A. $5,0,0,+1/2$
• B. $5,1,0,+1/2$
• C. $5,1,1,+1/2$
• D. $6,0,0,+1/2$

Answer 1

The correct option is A $5,0,0,+1/2$

The atomic number of rubidium can be found by the fact that 4 periods are fully filled and Rb is the first element in period 5 i.e., 2 + 8 + 8 + 18 + 1 i.e. 37
The valence shell configuration of atomic number 37 is:
$\begin{array}{ccc}{}_{36}\text{[Kr]}5s^1& \therefore & n=5\\ & & l=0\\ & & m=0\\ & & s=+1/2\text{or}-1/2\end{array}$