-In transition elements, the last shell contains one or two electrons. Penultimate shell may contain more than eight and up to eighteen electrons.

How many electrons are in penultimate shell?

Penultimate shell may contain eight or nine electrons and pre-penultimate shell contains more than 18 up to 32 electrons. enultimate shell means the 2nd last shell in which electrons are filled in an atomic orbital.

How do you calculate the number of penultimate shells?

If the valence shell has value 'n' then the penultimate shell will have value (n-1). Example :- The Magnesium has atomic number 12. It is a block element. So, the 3s is the valence shell here and 2p is the penultimate shell.