What is the principal quantum number for the outermost electrons in a Ca atom in the ground state?

##n=4##

The principal quantum number, ##n##, designates the energy level on which an electron is located.

The thing to remember about the energy level that holds an outermost electron in an atom in its ground state is that it corresponds to the period in which the element is located in the Periodic Table of .

In this case, calcium, ##"Ca"##, is located in period ##4## of . As you know, the periods are the horizontal rows.

This means that for a calcium atom in its ground state, the outermost electrons are located on the fourth energy level, which, of course, implies that

##n = 4##