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A radioactive nucleus has 90 protons and 140 neutrons. It undergoes two successive alpha decays. What are the mass number and atomic number of the final nucleus?
- (a)Mass number = 222, atomic number = 86
- (b)Mass number = 226, atomic number = 88
- (c)Mass number = 224, atomic number = 86
- (d)Mass number = 222, atomic number = 88
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Mass number = 222, atomic number = 86
Reason — For the given nucleus,
Atomic number Z = number of protons = 90
Mass number A = number of protons + number of neutrons = 90 + 140 = 230
An alpha particle is a helium nucleus 4He2, so each alpha emission decreases the mass number by 4 and the atomic number by 2.
For two successive alpha decays,
New mass number = 230 − (2 × 4) = 230 − 8 = 222
New atomic number = 90 − (2 × 2) = 90 − 4 = 86
Hence, the final nucleus has mass number 222 and atomic number 86.