Question 5
A nucleus X decays
into a nucleus Y by emitting an alpha particle followed by two beta particles.
Which statement about
this nuclear decay is correct?
A Beta
particle decay occurs when a proton changes into a neutron.
B Nucleus
Y has the same nucleon number as nucleus X.
C Nucleus
Y is an isotope of nucleus X.
D The total mass of the products is equal to the
mass of the initial nucleus X.
Reference: Past Exam Paper – June 2014 Paper 12 Q38
Solution:
Answer:
C.
From the information give,
the decay can be represented as:
X - - - > Y + 42α +
2 0-1β
Consider
option A: Beta particle decay
occurs when a proton changes into a neutron.
11p - - - > 10n + 0-1β
Beta particle has a
negative charge.
Considering the charges:
Left-hand side: charge =
+1 Right-hand side: charge =
0 + -1 = -1
This is incorrect as
charge is not conserved.
Consider
option B: Nucleus Y has the same
nucleon number as nucleus X.
Let the nucleon number of
nucleus Y be y and nucleon number of nucleus X be x.
Beta emission does not
change the nucleon number.
Consider the nucleon
number:
x = y + 4 + 2(0)
x = y + 4
y = x – 4
So, x is not equal to y.
Consider
option C: Nucleus Y is an isotope
of nucleus X.
Isotopes of an element are
nuclei having the same proton number.
X - - - > Y + 42α +
2 0-1β
Considering the proton
numbers:
X = Y + 2 + 2(-1)
X = Y
This is correct as there
is no overall change in the proton number.
Consider
option D: The total mass of the
products is equal to the mass of the initial nucleus X.
For any nuclear reaction /
decay, there should be a mass defect which results in the release of energy.
So, the total mass is not conserved as some of it is converted into
energy. However, mass-energy is conserved.
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