Question

Which of the following species is isotonic with ${ }_{37} Rb ^{86}$ ?

• A. ${ }_{38} Sr ^{87}$
• B. ${ }_{36} Kr ^{84}$
• C. ${ }_{39} Y ^{89}$
• D. ${ }_{37} Mg ^{85}$

Answer 1

Answer: (A)

Isotonic species are those which have identical numbers of neutrons.

$37 R b^{86} ; \,\,\,\,\,\, n=86-37=49$

$38 Sr ^{87} ; \,\,\,\,\,\, n =87-38=49$

$36 Kr ^{84} ; \,\,\,\,\,\, n =84-36=48$

$39 Y ^{89} ; \,\,\,\,\,\,\,\,\,\, n =89-39=50$

$37 Mg ^{85} ; \,\,\,\,\,\, n =85-37=48$