Multiple Choice
Which statement best defines a radioactive isotope?
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4. Atoms and Elements
Isotopes
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How many protons are in an atom of strontium-90?
A
128
B
38
C
90
D
52
Verified step by step guidance1
Identify the element in question, which is strontium. Each element is defined by its atomic number, which is the number of protons in its nucleus.
Recall that the atomic number of strontium (Sr) is 38. This means every atom of strontium has 38 protons regardless of its isotope.
Understand that the number 90 in strontium-90 refers to the mass number, which is the sum of protons and neutrons in the nucleus, not the number of protons alone.
Since the number of protons is fixed for an element, the number of protons in strontium-90 is the atomic number of strontium, which is 38.
Therefore, to find the number of protons in any isotope of strontium, simply use the atomic number 38, ignoring the mass number.
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