Calculating neutrons from ion's electron count, charge, and mass number Steps:

• Electronic configuration of 2,8,8 indicates 18 electrons (2 + 8 + 8) in the ion.
• For a 2– ion, atomic number Z = electrons – charge magnitude = 18 – 2 = 16 (protons).
• Nucleon number is mass number A = protons + neutrons = 36.
• Neutrons = A – Z = 36 – 16 = 20. Why D is correct:
• Neutrons equal mass number minus atomic number, where Z = 16 from 18 electrons adjusted for 2– charge (electrons = Z + |charge|). Why the others are wrong:
• A: 16 neutrons implies Z = 20; 2– ion would have 22 electrons (configuration 2,8,12), not 2,8,8.
• B: 18 neutrons implies Z = 18; 2– ion would have 20 electrons (configuration 2,8,10), not 2,8,8.
• C: 19 neutrons implies Z = 17; 2– ion would have 19 electrons (configuration 2,8,9), not 2,8,8.

Final answer: D