Alpha particles are

In the nuclear reaction $_{85}X^{297} \to Y + 4\alpha$,$Y$ is:

$A$ free neutron decays into a proton, an electron and

$A$ radioactive element $A$ decays into radioactive element $C$ by the following processes in succession.
$A \rightarrow B + {}_{2}^{4}He$
$B \rightarrow C + 2e^{-}$
Then elements

Radioactivity is a/an ...... process.

The ${ }_{92}^{235} U$ atom disintegrates to ${ }_{82}^{207} Pb$ with a half-life of $10^9 \ yr$. In the process,it emits $7 \ \alpha$ particles and $n \ \beta^{-}$ particles. Here,$n$ is