The outermost shell of an atom with 15 electrons is called the third energy level. The capacity of any energy level is equal to 2n^2, where n is the energy level. Therefore, the third energy level can hold a maximum of 2(3)^2 = 18 electrons. An atom with 15 electrons has 2 electrons in the first shell and 8 electrons in the second shell. This means that the third shell of this atom will have 5 electrons.