91Ó°ÊÓ

We have been given that,

Two stable nuclei.

We need to find the smallest value of $A$and two stable nuclei.

The smallest value of $A$for which nuclei is stable is$12and16$.

The two stable nuclei are :

$$\begin{gathered} C^{12}:A=12 \\ {O}^{16}:A=16 \end{gathered}$$

Stable nuclei cluster very close to the curve called 'line of stability' .

From the curve ,we see that the familiar element $C^{12},O^{16}$have equal number of protons and neutrons.

Also,

$Z<16$ when $N≈Z$ These elements are stable .

We have been given that,

No stable nuclei.

We need to find the value$A$such that less than this there is no stable nuclei .

The value of $A$less than this no stable nuclei is =$4$.

The nuclei is ,

$H{e}^4:A=4$

From the stability curve ,we clearly see that there is no stable nuclei less than $A=4$.