Which of the two elements would be more reactive, element A of atomic number 18 or element B of atomic number 19?

Explanation

The electronic configurations of elements A and B are given below:
`{:(18^(A),2,8,8,),(19^(B),2,8,8,1):}`
Since, the outermost shell of element A is completely filled, it is stable and unreactive. The outermost shell of element B contains one electron only. Hence, it is more reactive.