June 2018 NY Regents Chemistry Exam
36 What is the total number of neutrons in an atom of K-42? (1) 19
Solution: Mass number = number of protons + number of neutrons. The mass number of K is given to us in the question and is 42. Next, the number of protons is equal to the atomic number and can be found on the Periodic Table. The atomic number of K is 19. Now we can find the number of neutrons.
42 = 19+x
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