In the following table, which letter represents an ion? [table]
Isotope | Proton number | Neutron number | Electron number V | 19 | 20 | 19 W | 20 | 22 | 20 X | 20 | 22 | 18 Y | 18 | 22 | 18 Z | 18 | 20 | 18
[/table]

Question

BlackTom AI · Accepted Answer

An ion is formed when the number of electrons does not equal the number of protons; the net charge equals (protons − electrons) in units of the elementary charge (e).

Step-by-step check each entry (protons p, electrons e, net charge = p − e):

- V: p = 19, e = 19 → net charge = 19 − 19 = 0 → neutral atom (not an ion).
- W: p = 20, e = 20 → net charge = 20 − 20 = 0 → neutral atom.
- X: p = 20, e = 18 → net charge = 20 − 18 = +2 → this is a cation with charge +2 (an ion).
- Y: p = 18, e = 18 → net charge = 18 − 18 = 0 → neutral atom.
- Z: p = 18, e = 18 → net charge = 18 − 18 = 0 → neutral atom.

Intermediate calculation shown for X: net charge = 20 − 18 = +2, meaning X carries a net positive charge of +2e (written as X2+).

Why X is correct: it is the only entry where proton and electron counts differ, producing a nonzero net charge and therefore an ion. The others are neutral because their proton and electron numbers are equal.

Why the other options are incorrect:
- V (option c) has equal protons and electrons (19 each) → neutral.
- W (option b) has equal protons and electrons (20 each) → neutral.
- Y (option d) has equal protons and electrons (18 each) → neutral.

Conclusion: Only isotope X is an ion (specifically a +2 cation), so the correct choice is a. X, matching the provided answer.

In the following table, which letter represents an ion? [table] Isotope | Proton number | Neutron number | Electron number V | 19 | 20 | 19 W | 20 | 22 | 20 X | 20 | 22 | 18 Y | 18 | 22 | 18 Z | 18 | 20 | 18 [/table]单项选择题

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