# In mercury the o shell has 18 electrons but we know that second last shell cannot have more than 8 electrons? [closed]

The last shell has 2 electrons and the second last has eighteen electrons. Byt we know that second last shell cannot have more than 8 electrons. Then how is this possible. Please explain in simple manner.

• How do you know that the second half-shell cannot have more than $8$ electrons ? Is this rule valid ? Never seen before.. Jan 17 at 15:14
• It depends if by a shell is meant orbitals grouped by similar energy or the same main quantum number. It is 18 for the former but 8 for the latter. E.g. for Rb with 5s1 electron, there are 8 electrons with main quantum number 4 but 18 if 3d electrons are involved energetically. For mercury, energetically, there come also 14 4f electrons. Jan 17 at 15:21

This is dictated by the manner in which shells and subshells are filled in typical ground state configurations of neutral atoms. Take the $$n=3$$ shell. It can hold a total of $$2×3^2=18$$ electrons, but only eight are there in argon and then when we move to potassium and calcium, the $$4s$$ orbital fills before we get any electrons into the $$3d$$ subshell. Then the last shell becomes $$n=4$$ instead of $$n=3$$. Only after it becomes the second to last shell does $$n=3$$ gain ten more electrons (filling the $$3d$$ subshell) as we go from scandium to zinc, and then it gets to eighteen.