How many electrons are in the valence d orbitals in these transition-metal ions? ?

There are Cd^2+ electrons in the valence d orbitals in these transition-metal ions. The valence electron is a type of electron which is connected to the atom and it plays an important role in the process of the chemical bonding.

3 hours because it is a decent distance.

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.