Question

c) M shell can have upto 18 electrons. Then why Potassium (19) does not have the
electronic configuration 2,8,9.​

Answer 1

It is a rule according to the energy of the electrons

Up till atomic no. 20 the electronic configuration will be like 2,8,8,2

From atomic no 21 the electronic configuration will be like 2,8,9,2 and so on