Q133
General ScienceChemistry
The ion of an element has three positive charge. Mass number of the atom is 27, and the number of neutrons is 14. What is the number of electrons in the ion?
- a.13
- b.10
- c.14
- d.16
Answer: (B) 10
Number of protons (atomic number) = mass number − neutrons = $27-14=13$, so the element is aluminium. A neutral atom has 13 electrons; a charge of $+3$ means three electrons have been lost, leaving $13-3=10$. The configuration changes from 2, 8, 3 to 2, 8, so $Al^{3+}$ is isoelectronic with neon and with $Na^+$, $Mg^{2+}$, $F^-$ and $O^{2-}$. Option (A) 13 is the number of protons or of electrons in the neutral atom, and (C) 14 is the number of neutrons. (The booklet prints "change" for "charge".)