How do substances change after a chemical reaction

The atoms re orient themselves after a chemical reaction

Explanation:

In nature all the chemical substance cannot exists in their atomic form. They can exists freely only in molecular form. In order to do so these atoms of an element either form group among themselves or with some other elements thereby leading to a chemical reaction.  

For example – a single atom of oxygen O2- is unstable in the nature and thus it react with itself to form O2 molecule. The two oxygen atom forms a covalent bond between them thereby restructuring its orientation.  

Like wise all other chemical element also align themselves into a new structure in order to form a new compound after a chemical reaction.  

the sugar-phosphate backbone forms the structural framework of nucleic acids, including dna and rna. this backbone is composed of alternating sugar and phosphate groups, and defines directionality of the molecule.

the correct answer, according to e2020, is the second choice there is an occluded front in region a and a cold front in region b.