Physics Nuclear Physics MDCAT 2012
PMDC Verified Question 85 of 98
A radioactive nuclide decays by emitting an \(\alpha\)-particle and a \(\gamma\)-ray photon, the change in the nucleon number will be:
A
-4
B
-2
C
+2
D
-3
Tap any option to test your recall and reveal the step-by-step Propolis autopsy.

Propolis Cognitive Error Autopsy

Official Correct Choice:
Option A: -4


Concept:

The "nucleon number" is another term for the mass number (\(A\)), which is the total count of protons and neutrons in a nucleus.

Solution:

  • An \(\alpha\)-particle is a helium nucleus (\(_{2}^{4}\text{He}\)), meaning it carries away 4 nucleons (2 protons + 2 neutrons). This decreases the parent's nucleon number by 4.


  • A \(\gamma\)-ray is a high-energy photon. It carries away excess energy but has zero mass and zero charge, meaning it removes 0 nucleons.


  • Total change: \(\Delta A = -4 + 0 = -4\).


Why other options are incorrect:

Option B is the change in the atomic number (\(Z\)), not the nucleon number. Options C and D do not match standard decay rules.

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