How many electrons does the ion ²⁷₁₃Al³⁺ have?

Correct answer: a 10

In a positive ion, electrons = Z − charge = 13 − 3 = 10.

Why the other options are wrong

• b 13: 13 is the proton count; a charge of 3+ means three fewer electrons.
• c 14: 14 is the neutron count (27 − 13); the electrons are 10.
• d 27: 27 is the mass number; the electrons are Z − 3 = 10.