Question detail
If an atom has 15 protons, 16 neutrons, and 15 electrons, what is its mass number?
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Atomic structure
Question
- A. 15
- B. 31
- C. 16
- D. 1
Answer
31
Explanation
The mass number is the sum of protons and neutrons. Here, mass number = protons + neutrons = 15 + 16 = 31.
Common mistake
Misidentifying Neutrons from Mass Number
Students often subtract the atomic number from the mass number to find the number of neutrons, but then forget to adjust for the ion’s charge when calculating electrons.
Remember that the number of neutrons is always mass number minus atomic number, independent of charge. The electron count is atomic number minus the ion’s charge (negative charge means +1 electron, positive charge means –1 electron).
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