Isotonic symbol of the ion of Sulphur-33 is shown below. How many protons and neutrons are present if the number of electrons are 18?
Correct answer: B. p=16, n=17
- A. p= 18, n= 15
- B. p=16, n=17
- C. p=16, n=16
- D. p=17, n=16
Explanation
Sulphur has atomic number 16, so its nucleus holds 16 protons whatever the ion's charge. Mass number 33 gives neutrons = 33 - 16 = 17. The 18 electrons only tell you the atom has gained two electrons to form the sulphide ion; gaining electrons never changes the proton or neutron count.
This question appeared on the UHS MDCAT 2017 paper, which you can sit online with every answer explained.
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