CBSE Class 9 Answered

According to Bohr's theory of distribution of electrons, the outermost shell of an atom cannot accomodate more than 8 electrons, even if it has the capacity to hold more electrons. For example in case of Magnesium; its atomic number is 12 so the electronic configuration will be: K-2, L-8, M-2. Now L shell has maximum capacity of 8 electrons only so, the 2 electrons left will go to M shell though it has tendency to hold more electrons. In case of Calcium, it belongs to S-block and has the electronic configuration 1s²2s²2p⁶3s²3p⁶4s²  

 

i.e. it has 4 shells in it so its configuration is: K-2, L-8, M-8, N-2; although M shell can accomodate upto 18 electrons.