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9th Class Science Structure of the Atom Question Bank Atomic Structure Conceptive Objective Problems

  • question_answer
    An atom has net charge of -1. It has 18 electrons and 20 neutrons. Its mass number is

    A)  37                         

    B)  35                         

    C)  38                                         

    D)  20   

    Correct Answer: A

    Solution :

        No. of electrons in uni-negative ion = 18 No. of neutrons                                                                                                   \[(n)=20\] Mass number                                                                                                    \[(A)=?\]                                                                                                   \[A=Z+n\] ____________ (1) Z = No. of protons (or) No. of electrons in neutral atom. No. of electrons in neutral atom                                                                                                 \[=181=17\] _________ (2) Substituting 2 in 1, we get,                                                                                             \[A=17+20=37.\]


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