The atoms ,molecules, or compound present at the start of a chemical reaction that parcitipate in the reaction . which part of a chemical reaction does this define?

The atoms ,molecules, or compound present at the start of a chemical reaction that parcitipate in the reaction are the reactants.

Explanation:

The chemical reaction is the way in which one substance reacts against another. So, a chemical reaction consists of the transformation of some substances into others, that is, the process of arranging atoms and bonds when chemical substances come into contact.

In a chemical reaction, the initial substances are called reactants, while the new substances obtained are called products.

So, the atoms ,molecules, or compound present at the start of a chemical reaction that parcitipate in the reaction are the reactants.

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a.   mass number from atomic number