Assuming a zinc atom has mass number 64, how many neutrons are present in a dipositive zinc ion Zn2 plus, given that ionisation does not change the nucleus?

Introduction / Context:
This chemistry question tests basic nuclear composition concepts, namely the relationship between atomic number, mass number, and number of neutrons. It also checks whether learners understand that forming an ion by losing or gaining electrons does not affect the protons or neutrons in the nucleus. Zinc is a common example used in such questions, and here we assume a specific mass number to make the calculation clear and unambiguous.

Given Data / Assumptions:
- The species considered is a dipositive zinc ion Zn2 plus.
- The mass number of zinc is taken as 64 for this question.
- The atomic number of zinc is 30, which means zinc has 30 protons.
- Ion formation changes electrons, not the nucleus, so the number of neutrons stays the same as in the neutral atom.

Concept / Approach:
For any nuclide, mass number is the sum of protons and neutrons. Atomic number is the number of protons and defines the element. The number of neutrons can be calculated using the simple relation: number of neutrons = mass number minus atomic number. When an atom forms a positive ion, it loses one or more electrons, but the protons and neutrons in the nucleus are unchanged. Therefore, the neutron count in Zn2 plus is the same as in neutral zinc of mass number 64.

Step-by-Step Solution:
Step 1: Note that zinc has atomic number 30, so every zinc atom or ion has 30 protons in its nucleus.
Step 2: The question specifies that the mass number of zinc to be used is 64. The mass number is the total of protons and neutrons.
Step 3: Use the relation number of neutrons = mass number minus atomic number. Therefore, number of neutrons = 64 minus 30.
Step 4: Perform the subtraction: 64 minus 30 gives 34 neutrons.
Step 5: Recognise that forming a Zn2 plus ion means zinc has lost two electrons, but these electrons are not counted in mass number. Protons and neutrons remain unchanged, so the ion still has 34 neutrons.

Verification / Alternative check:
A quick check is to remember that typical isotopes of zinc include Zn 64, Zn 66, Zn 68, and so on. For Zn 64, the difference between 64 and the atomic number 30 must be the neutron count, which is 34. Some learners remember this from isotope tables, but even without memorising, the simple subtraction method always works. Checking that 34 plus 30 equals 64 confirms that the calculated neutron number is consistent with the mass number given.

Why Other Options Are Wrong:
- 36: This would imply a mass number of 66 for zinc if the atomic number is 30, which is not the isotope specified in the question.
- 38: This would match an isotope with mass number 68, not the given mass number 64.
- 40: This would correspond to a mass number of 70, again different from the assumed mass number 64.

Common Pitfalls:
A common error is to think that the charge on the ion somehow affects the number of neutrons. Learners might incorrectly add or subtract the charge value from the neutron count. Another mistake is to confuse mass number with atomic mass or to forget the atomic number of zinc. Finally, some students may subtract in the wrong order and compute 30 minus 64 instead of 64 minus 30. Carefully applying the relation mass number = protons plus neutrons and remembering that electrons are not included in mass number avoids these problems.

Final Answer:
34 neutrons are present in a Zn2 plus ion when zinc has a mass number of 64.