Question

The isotope which has the greatest number of neutrons is :

• A. ${}^{35}Cl$
• B. ${}^{31}P$
• C. ${}^{40}Ar$
• D. ${}^{41}Ca$
• E. ${}^{41}C$

Answer 1

The correct option is C ${}^{40}Ar$

The mass number is the sum of protons and neutrons in the nucleus.

The atomic number is the number of protons. Therefore, if the atomic number is subtracted from the mass number, it gives the number of neutrons.

Atomic number of ${}^{35}Cl=17\Rightarrow$ Number of neutrons$=35-17=18$

Atomic number of ${}^{31}P=15\Rightarrow$ Number of neutrons$=31-15=16$

Atomic number of ${}^{40}Ar=18\Rightarrow$ Number of neutrons$=40-18=22$

Atomic number of ${}^{41}Ca=20\Rightarrow$ Number of neutrons$=41-20=21$

Atomic number of ${}^{14}C=6\Rightarrow$ Number of neutrons$=14-6=8$

${}^{40}A{r}_{18}$ has greatest number of neutrons.

Hence option C is correct.