In radioactive decay, the nucleus changes. It is important to realise that both the nucleon number and the proton number are conserved in the reaction.
We have already established that an α-particle is a helium nucleus and can be represented as:

$\begin{array}{*{20}{c}}
4\\
2
\end{array}\alpha $

The isotope radon-222 decays by $\alpha  - $emission, we can describe this by the equation:

$\begin{array}{*{20}{c}}
{222}\\
{86}
\end{array}Rn \to \begin{array}{*{20}{c}}
{218}\\
{84}
\end{array}Po + \begin{array}{*{20}{c}}
4\\
2
\end{array}\alpha $

A quick glance tells us there are 222 nucleons before the decay and $218 + 4 = 222$ after the decay.
Similarly, there are 86 protons before and $84 + 2 = 86$ after the decay.
The isotope $Mn - 56$ decays by ${\beta ^ - }$ emission:

$\begin{array}{*{20}{c}}
{56}\\
{25}
\end{array}Mn \to \begin{array}{*{20}{c}}
{56}\\
{26}
\end{array}Fe + \begin{array}{*{20}{c}}
0\\
{ - 1}
\end{array}\beta  + \begin{array}{*{20}{c}}
0\\
0
\end{array}\overline V $

Again, it is easy to see the nucleons are conserved; however, we need to recognise that the electron is regarded $as −1$ proton.
Note also that an antineutrino is also emitted; interestingly, this suggests the number of particles/antiparticles are the same before and after the decay.
For the ${\beta ^ + }$ decay we look at the isotope $V-48$:

$\begin{array}{*{20}{c}}
{48}\\
{23}
\end{array}V \to \begin{array}{*{20}{c}}
{48}\\
{22}
\end{array}Ti + \begin{array}{*{20}{c}}
0\\
1
\end{array}\beta  + \begin{array}{*{20}{c}}
0\\
0
\end{array}v$

Once again there is a balance of proton numbers, nucleon numbers and particles/antiparticles (remember that the positron is an antiparticle).
There is another quantity that is conserved. You might expect mass to be conserved, but this is not so. For example, in the $\alpha $ decay equation given previously, the combined mass of the polonium nucleus and the alpha particle is slightly less than that of the original radon nucleus. The ‘lost’ mass has become energy – this is where the fast-moving alpha particle gets its kinetic energy. The relationship between mass m and energy E is given by Einstein’s equation $E = m{c^2}$, where c is the speed of light in free space. So, instead of saying that mass is conserved in nuclear processes, we have to say that mass–energy is conserved.
There is much more about this in Chapter 29.