Alpha decay occurs for heavy isotopes. In alpha decay the nucleus loses a helium nucleus.

A helium nucleus is very stable so is preferrentially lost to gain stability.

$$ ^{238}_{92}U \longrightarrow ^{234}_{90}\text{Th} + ^4_2\text{He} $$

In this decay it is easy to predict the daughter nuclei as the nucleons and protons are conserved.

$$ A \longrightarrow A-4 \text{ and } 4 $$

$$ Z \longrightarrow Z-2 \text{ and } 2 $$

energy is released as a small amount of mass is turned into energy.

Beta decay occurs for light isotopes. In beta minus decay the nucleus converts a neutron into a proton in the nucleus.

A neutron is heavier than a proton so this makes this energetically favourable.

An electron and an anti-electron neutrino are emitted as well.

$$ ^{14}_{6}\text{C} \longrightarrow ^{14}_{7}\text{N} + ^0_{-1}e^- + \overline{\text{n}} $$

When an isotope decays its very likely to be an excited state after the decay.

Think of this like an atomic excited state. The nucleus is similar and has excited states.

To return to the nuclear ground state the nucleus emits a high energy photon.

These photons are in the gamma wavelength range in the e/m spectrum.

No structural change occurs.