How does the structure of a carbon atom enable it to form large molecules

 Because of the property known as Catenation.

Explanation:  Catenation is the property of making molecules by self linkage.

This is the property mostly exhibited by the carbon. Thus it is famous for creating large molecules or large chains of carbon and hydrogen atoms.

This property is also exhibited sulfur of the chalcogen family.

The structure of a carbon atom enables it to form large molecules as it can form four chemical bonds to the other atoms as well. It usually forms covalent bond by sharing of the electrons.

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.