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Q.
Which of the following is most unstable?
AFMCAFMC 2005Atoms
Solution:
(a) Electron revolves around the nucleus of an atom. lt is a stable particle.
(b) Proton has positive charge and its mass is 1836 times the electronic mass. In free state, the proton is a stable particle.
(c) Neutron carries no charge. lts mass is 1839 times the electronic mass $(1.675 \times 10^{-27}kg).$ In free state the neutron is unstable (its mean life is about $17\, \min$), but it constitutes a stable nucleus along with proton.
(d) $\alpha -$ particle is helium nucleus whose atomic number is $2$ and mass number is $4$.
Hence, out of the given four options neutron is most unstable.