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207. Which isotope has the same number of neutrons as calcium-42?
     
Mass Number = number of protons  +  number of neutrons

number of neutrons = Mass Number - number of protons

Atomic Number = number of protons    (Look on Periodic Chart)

Calcium-40    number of neutrons = 42 - 20 = 22
    
A. calcium-40 number of neutrons = 40 - 20 = 20
    
B. potassium-41 number of neutrons = 41 - 19 = 22    The Correct Answer is B.
    
C. scandium-42 number of neutrons = 42- 21 = 21
     
D. argon-41 number of neutrons = 41 - 18 = 23