NY Chemistry Regents Exam June 2019

Questions 62-65

Cobalt-60 is an artificial isotope of Co-59. The incomplete equation for the decay of cobalt-60, including beta and gamma emissions, is shown below.

62 Explain, in terms of *both *protons and neutrons, why Co-59 and Co-60 are isotopes of cobalt. [1]

**Solution: **Isotopes are atoms of the same element that have the same number of protons but a different number of neutrons.

**Answer**: Co-59 and Co-60 are isotopes of cobalt because they have the same number of protons but a different number of neutrons

63 Compare the penetrating power of the beta and gamma emissions. [1]

**Solution and answer: **Gamma has a higher penetrating power than beta.

64 Complete the nuclear equation, *in your answer booklet*, for the decay of cobalt-60 by writing a notation for the missing product. [1]

**Solution: **In order to find the missing product, we need to know that the sum of masses (top) on the left of the equation is equal to the the right. 60 = x+0+0, x=60

We do the same for the bottom number (atomic number). 27 = y+ -1+0

y=28. We find the element with atomic number of 28 on the Periodic Table and it is Ni

**Answer:** Ni-60

65 Based on Table *N*, determine the total time required for an 80.00-gram sample of cobalt-60 to decay until only 10.00 grams of the sample remain unchanged. [1]

**Solution: **Table N on the Reference Table provides half lives for different isotopes. The half like for cobalt-60 is 5.271 years. Half life represents the amount of time it take something to become a half of the original amount. We need to go from 80 g to 10 g. Let's see how many half lives it is.

80 -> 40 -> 20 -> 10

This is 3 half lives or 3*5.271 = 15.813 y

**Answer: **15.813 y