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

Please find answer in explanation column

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.

2h2o2--> 2h2o + o2

this reaction is of the spontaneous decomposition of hydrogen peroxide down into water and oxygen.

add 2 molecules of hydrogen peroxide and 2 molecules of water. since oxygen is naturally diatomic, the total number of atoms of each element is now the same on both sides of the equation so it is balanced.

this may not be entirely correct but let me know.

"record the time to complete a chemical reaction" would most likely be part of a lab procedure.