Discussion
The ion of an element has two positive charges. The mass number of the atom is 27 and the number of neutrons is 14. What is the number of electrons in the ion?
Answer: C
The ion has 11 electrons.
The number of protons is the mass number minus the number of neutrons. A neutral atom has as many electrons as protons, and a charge of 2+ means two electrons have been lost.
\[ \begin{aligned} \text{protons} &= 27 - 14 = 13 \\ \text{electrons in ion} &= 13 - 2 = 11 \end{aligned} \]
13 is the number of electrons in the neutral atom, 15 comes from adding the two instead of subtracting, and 10 is the result of subtracting three electrons, the charge of the familiar \(\mathrm{Al^{3+}}\) ion, instead of the two stated here.
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