Question detail
A chlorine ion (Cl-) has an atomic number of 17 and a mass number of 35. How many protons, neutrons, and electrons does it have?
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. 17 protons, 18 neutrons, 17 electrons
- B. 17 protons, 18 neutrons, 18 electrons
- C. 18 protons, 17 neutrons, 17 electrons
- D. 17 protons, 17 neutrons, 17 electrons
Answer
17 protons, 18 neutrons, 18 electrons
Explanation
The atomic number (17) indicates the number of protons. The mass number (35) is the sum of protons and neutrons. Therefore, neutrons = mass number - atomic number = 35 - 17 = 18. Since the ion has gained one electron, the number of electrons is 17 + 1 = 18.
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).
Related flashcards
Flashcard 1 of 5
Related practice questions
Question 1 of 5
Choose an answer, get feedback, then move sideways through the set.