During radioactive decay, the isotope decays into a isotope that has a different number.

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Explanation:

During radioactive decay, the __unstable isotope decays into a _stable isotope that has a different proton number

Or

During radioactive decay, the _ unstable parent nuclide isotope decays into a _stable daughter nuclide isotope that has a different proton number.

There are 3 types of radioactive decay;alpha, beta and gamma, Of which the above clearly explains the beta decay. In beta decay, the unstable isotope having excess neutrons  will undergo a beta decay emitting a beta particle.( ⁰₋₁e) causing the nucleus to loose a neutron but gain a proton.

Some heavy unstable isotopes which undergo radioactive (beta decay ) to become stable isotopes  are phosphorus-32, strontium-90, iodine-131

Using Strontium 90 as an example , we have

⁹⁰₃₈St >⁹⁰₃₉Y + ⁰₋₁e

Strontium an unstable isotope undergoes a beta radioactive decay to form Yttrium.

it would be d. henry moseley

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