Question

An element is represented by ${}_Z{X}^A$, where Z is atomic number which is also equal to number of proton, A is mass number and X is symbol of the element.
Mass number (A) = Number of protons (Z) + Number of neutrons.
Isotopes: Atoms having same number of proton
Isobar: Elements having same mass number
Isotones: Elements having same number of neutron (A-Z)

An isotone of ${}_{40}^{90}Zr$ is _________.

• A. ${}_{36}^{86}Kr$
• B. ${}_{19}^{39}K$
• C. ${}_{35}^{80}Br$
• D. ${}_{50}^{119}Sn$

Answer 1

The correct option is A ${}_{36}^{86}Kr$

Isotones are species that have the same number of neutrons but different number of protons.
Number of neutrons = mass number – atomic number
Number of neutrons in ${}_{40}^{90}Zr=90–40=50$

Number of neutrons in ${}_{36}^{86}Kr=86–36=50$
Number of neutrons in ${}_{19}^{39}K=39–19=20$
Number of neutrons in ${}_{35}^{80}Br=80–35=45$
Number of neutrons in ${}_{50}^{119}Sn=119–50=69$

Therefore, ${}_{36}^{86}Kr$ is an isotone of ${}_{40}^{90}Zr$.