∵ Effective nuclear charge =2.60 and valency shell contain 3 electrons. Thus, minimum number of main shells for the given element are two i.e (n=2) and its configuration will be 1s2,2s22p1 Thus, the given element has 5 electrons in all. Also, for a neutral atom. ∵ No. of electrons = Atomic number Thus, atomic number of the element is 5 .