Question

Isodiapheres are atoms having:
[A] $p\div n$constant
[B] (p-n) constant
[C] (n-p) constant
[D] (n-p) different

Answer 1

HINT: Isodiaphers are the atoms of different elements having the same isotopic number. They may have different atomic mass and atomic numbers but the difference between the atomic mass and twice the number of protons remains the same.

Complete step by step solution:

Isodiaphers are nuclides of different elements having the same excess of neutrons over protons (A-2Z)
A= Atomic mass
Z=Number of protons
For example, $^{37}C{{l}_{17}}$and $^{39}A{{r}_{18}}$
For Cl, Number of neutrons=37-17=20
Number of protons=17
For Ar, Number of neutrons=39-18=21
Number of protons= 18
As we can see,
Chlorine has (20-17) =3 excess neutrons
Argon has (21-18) =3 excess neutrons
Both Chlorine and Argon have 3 neutrons in excess over the protons therefore, they are considered as isodiaphers.
Also, For Cl,
(A-2Z)= 37-(2$\times$17) =3
For Ar,
(A-2Z)= 39-(2$\times$19) =3
The (A-2Z) value is the same for both Cl and Ar, making them isodiaphers.
Which means, their (n-p) value is constant.
Therefore, option [C] (n-p) constant is the correct option.
ADDITIONAL INFORMATION: Nuclide and its decay products after $\alpha$-emission are also isodiaphers. Other examples of isodiaphers are $_{29}C{{u}^{65}}$and $_{24}C{{r}^{55}}$, Thorium-234 and Uranium-238.

NOTE: Isodiaphers might be confused as isobars or isotones. But it is important to keep in mind that isobars are compounds with same mass number but different atomic numbers I.e.they have different number of protons and isotopes are nuclides with same number of neutrons but different mass numbers and isodiaphers are nuclides of different elements having the same number of excess neutrons.