The distribution of electrons In K, L, M, N shells and the number of neutrons in the nuclei of the following elements are given below: ElementsKLMNNo. of NeutronsA26--8B282-12C285-16D288-22 The mass number of A, B, C, and W respectively is:
A
16, 24, 31, 40
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B
31, 24, 16, 40
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C
31, 40, 16, 24
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D
40, 24, 16, 31
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Solution
The correct option is A 16, 24, 31, 40 Total number of electrons in an element = No. of electrons in K shell + No. of electrons in L shell +No. of electrons in M shell +No. of electrons in N shell Total number of electrons in element A = 2 +6 +0 +0 = 8 Total number of electrons in element B = 2 +8 +2 +0 = 12 Total number of electrons in element C = 2 +8 +5 +0 = 15 Total number of electrons in element D = 2 +8 +8 +0 = 18 As we know total number of protons is equal to total number of electrons, therefore Total number of protons in element A = 8 Total number of protons in element B = 12 Total number of protons in element C = 15 Total number of protons in element D = 18 Atomic mass number = Total number of protons + Total number of neutrons Atomic mass number of element A = 8 + 8 =16 Atomic mass number of element A = 12 + 12 =24 Atomic mass number of element A = 16 + 15 =31 Atomic mass number of element A = 22 + 18 =40