briefly explain why the number of electrons circling the nucleus determines the physical and chemical properties of the Atom

Electrons are negatively charged particles that orbits the nucleus

Explanation:

Chemical and physical properties of elements are usually determined by the number of electrons they posses. When atoms combine, they do so in order to attain stability.

answer is: c. the experiment shows that the red substance experienced a chemical change.

this is chemical change (chemical reaction), because two new substances are formed (metalig liquid and gas), the atoms are rearranged and the reaction is followed by an energy change.

this is example of chemical decomposition.

chemical decomposition (analysis) is the separation of a single chemical compound into its two or more elemental parts or to simpler compounds.  

chemical decomposition is opposite of chemical synthesis.