Question

A certain negative ion $X^{-2}$ has in its nucleus $18$ neutrons and $18$ electrons in its extra nuclear structure. What is the mass number of the most abundant isotope of 'X'?

• A. $35.46$
• B. $32$
• C. $36$
• D. $39$

Answer 1

Answer: (B)

$32$

$X^{-2}$ has $18e^{-}$ this means that 'x' has

gained $2e^{-}$

hence 'X' has $16e^{-}$ which implies x has

16 protons

mass number $=$ no. of neutrons $+$ no. of protons

$=18+16=34$

atomic number $\left(Z\right)=$ no. of protons $=16$

implies element is sulphur

Using periodic table, we can say that both ${}_{16}{S}^{34}$

& ${}_{16}{S}^{32}$ exist.

As ${}_{16}{S}^{32}$ is present in high proportion in earth

must, so we can say that ${}_{16}{S}^{32}$ is the

most abundant isotope