If x is the number of electrons and y is the number of nucleons present in ³²₁₆S²⁻, then which one of the following is correct?
- A.x = 14 and y = 32
- B.x = 16 and y = 30
- C.x = 18 and y = 32
- D.x = 18 and y = 30
Correct answer
Explanation
The correct answer is C, x = 18 and y = 32. The sulphide ion has 16 + 2 = 18 electrons, and its mass number of 32 is its count of nucleons.
Sulphur has atomic number 16, so a neutral sulphur atom has 16 protons and 16 electrons. The charge 2− means it has gained two extra electrons, giving 18, the same as argon. Nucleons are the particles in the nucleus, protons plus neutrons, and their total is the mass number written at the top left: 32 (16 protons and 16 neutrons). Gaining or losing electrons never changes the nucleus, so the nucleon count stays 32.
A is wrong because 14 electrons would mean the ion had lost two electrons, which is S²⁺, not S²⁻.
B is wrong because 16 is the electron count of the neutral atom, and 30 nucleons is not this isotope.
D is wrong because the electron count is right but the nucleon count of 30 does not match the mass number 32.
Exam tip: electrons in an anion = Z + charge, in a cation = Z − charge; nucleons = mass number, unchanged by ionisation.
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