KEAM 2018 Physics and Chemistry Paper

∵ 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 .