An atom that gains one extra electron forms an with a 1- charge.

Explanation: When an atom gains/loses an electron, the atom becomes charged, and is called an ion. Gaining an electron results in a negative charge, so the atom is an anion.

It forms an anion

Explanation:

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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.

d.) the average kinetic energy of the gas particles is greater in container b because its particles move faster.