Of the following: H2(g); He(g); CO2(g); which would behave least like an ideal gas? Why?

The gas that least behaves as an ideal gas has been carbon dioxide.

The ideal gas has been defined as the gas with the least interatomic attraction and collision. The result has been the constituent pressure and volume arose from the collision of the sample to the walls.

The  gases that have the property of low interatomic attraction and exist as single molecules behaves as an ideal gas. In the given gases, there has been single atomic level in hydrogen and helium with weak van der wall forces.

The carbon dioxide being a complex structure with strong interatomic interactions least behaves as an ideal gas.

For more information about ideal gas, refer to the link:

link

why do the isotopes of a particular element behave differently in nuclear reactions but the same in chemical reactions? chemical reactions involve changes to electrons in an atom; isotopes do not differ in electrons, and so they react the same way during chemical changes.