Question

The anion ${\mathrm{X}}^{3-}$has 18 electrons and its mass number is 31.How many neutrons are present in the nucleus ?

• A. 18
• B. 8
• C. 31
• D. 16

Answer 1

The correct option is D

16

Explanation for the correct answer

Option D

• Anion ${\mathrm{X}}^{3-}$ has 18 electrons.
• Anion ${\mathrm{X}}^{3-}$ is formed by gaining 3 electrons.
• During the formation of an ion, the number of electrons will change. but there is no change in the number of protons and neutrons.
• The number of electrons in element $\mathrm{X}$ ,before the formation of ${\mathrm{X}}^{3-}$ ion =Number of electrons in ${\mathrm{X}}^{3-}$- 3 = 18 - 3 = $15$
• Number of electrons in X = $15$
• Number of protons in X = $15$
• Atomic number of X = Number of protons = $15$
• Mass number = $31$
• Number of neutrons = Mass number - Atomic number =$31-15=16$.

Conclusion: Hence option D is correct, anion ${\mathrm{X}}^{3-}$ containing 16 neutrons.