How can you tell for sure that a redox reaction has happened?

Determine the value of the oxidation numbers of atoms or ions on the reactant side and the product side.

Explanation:

A redox reaction involves some ions, and atoms losing electrons and other gaining them in the same reaction. When oxidation occurs, there is loss of electrons by an element thus its oxidation number increases; becomes more positive. When reduction occurs an element gains electrons and the oxidation  number becomes more negative; reduces.

If an element (s) behaves in this manner in the whole reaction, then be sure to deduce that a redox reaction has occurred.

- calcium has greater affinity to bond with electron receptors than hydrogen does and hence, it has the ability to replace hydrogen in most of its compounds.

- lithium has 2 valence electrons in the product because it can make only 1 bond with hydrogen. beryllium has 4 valence electrons in the product because it can only make 2 bonds. it has fewer electrons than neon and can react with electron donors. each hydrogen has 2 valence electrons, the same as helium.

- the covalent bond formed between carbon and hydrogen is polar because carbon is more electronegative than hydrogen and it attracts the bonded electron pair closer to itself, thus giving hydrogen a partial positive charge.

- a hydrogen bond is a relatively weak bond that hydrogen atoms make with the electronegative atoms nitrogen, oxygen or fluorine.

so the best choice is a.) 2 calcium (ca) atoms