An isotope undergoes radioactive decay. The new isotope that forms has an atomic number that is 2 less than the original isotope’s. Which kind of decay has occurred, and how do you know?
1)alpha decay because alpha particles have a large mass
2)beta decay because beta particles can have negative charge
3)alpha decay because alpha particles have two protons and two neutrons
4)gamma decay because gamma rays are photons

3)alpha decay because alpha particles have two protons and two neutrons

Explanation:

When a radioactive isotope undergoes a decay, a daughter nucleus or nuclei is/are formed. The mass number and atomic number of the daughter nucleus gives us an idea of the nature of radioactive decay that the parent nucleus underwent.

If the daughter mass number of the daughter nucleus is four units less than that of the parent nucleus and the atomic  number of the daughter nucleus is two units less than that of the parent, then such a decay is an alpha decay.

An alpha particle has two protons and two neutrons thus it has a mass number of 4 and an atomic number of two.

definition

                    entropy is a measure of the randomness or the extent of disorder of a chemical process.

                      enthalpy is a measure of the heat change of a reaction occurring at a constant pressure.

measurement units:

                        entropy is measured in joule per kelvin (jk⁻¹)   whereas enthalpy is measured in joule per mole (jmol⁻¹)

requirements:

                            entropy has no requirements or limits, and its change is measured by the division between the heat change of the chemical process and the temperature.

                              in contrast, enthalpy can only be related to the heat change of the reaction under standard conditions.