<span>The answer is curie.
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Reactants are what make up the product.
When a bond is broken that should be a type of reaction. When bonds are broken sometimes heat is released.
A lone oxygen atom has 6 electrons in its outer shell which is not very stable, whereas as full octet (8 outer shell electrons) is stable. In order to achieve this two oxygen atoms will share 4 electrons, each contributing 2 electrons. Since these electrons exist within the orbitals of both atoms, to oxygen atoms essentially achieve a full octet.
D — density
hope this helps !