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FBISE Paper 2019: MCQs and answers with explanation 1. Electronic configuration of a transition element X in +2 oxidation state is [Ar]3d 5 . What is the atomic number a) 25 b) 26 c) 27 d) 24 ... Answer is a) Explanation:To determine complete electronic configuration of an element, we must know the electronic configuraiton of nearest noble gas electronic configuration, whose atomic number increase as follow: 2(He),10(Ne),18(Ar),36(Kr),54(Xe),86(Rn),118(Og). A transition element X is in +2 oxidation state, it means it has two electrons less then its neutral atom. So, simply add this oxidation state to the electronic configuration in order to calculate total mumber of electrons. Total number of elecrons are 18 (Ar) +5 (3d) + 2 = 25. As number of electrons are equal to number proton in a neutral atom, so its number of protons (or atom number) is 25. show Answer ...