In potassium and calcium,the fillings of electrons in N-shell starts after M-shell has acquired 8 electrons although M-shell can accomodate upto18 electrons.why?
- First rule: The maximum number of electrons present in a shell is given by the formula 2n2, where ‘n’ is the orbit number or energy level index (1, 2, 3…). Hence, the maximum number of electrons that different shells can accommodate is as follows:
1. First orbit or K-shell can accommodate maximally 2 x 12= 2 electrons.
2. Second orbit or L-shell can accommodate maximally 2 x 22= 8 electrons.
3. Third orbit or M-shell can accommodate maximally 2 x 32= 18 electrons.
4. Fourth orbit or N-shell can accommodate maximally 2 x 42 = 32 electrons.
- Octet rule: The maximum number of electrons that can be accommodated in the outermost orbit is eight.
- Third rule: Electrons cannot be filled in the outer shell until the inner shells are completely filled. This means that shells are filled in a step-wise manner, starting from the inner shell.
So, in potassium and calcium elements, M-shell contains 8 electrons respectively and N shell contains 1 and 2 electrons respectively. because M-shell can aquire maximum 18 electrons but due to octet rule it has only 8 electrons in potassium and calcium. It can have 18 electrons only when N shell contain minimum 1 electron.
Hope this clears to you!