Final answer:
The formula for the compound that forms between rubidium and iodine is RbI, which is derived from the combination of the Rb+ cation and the I- anion to create a neutral ionic compound.
Step-by-step explanation:
To determine the formula for the compound that forms between rubidium (Rb) and iodine (I), we use the concept of Lewis symbols and rules for writing formulas for ionic compounds. Rubidium is in group 1 of the periodic table and has a single valence electron, which it can lose to form a Rb+ cation. Iodine is in group 17 of the periodic table and has seven valence electrons, needing one more to achieve a full octet, thus it can gain an electron to form an I− anion. Since the charges are equal and opposite, just one of each ion is needed to balance the charge, resulting in the formula RbI. This process completes the octet for iodine and empties the valence level of rubidium, creating a stable ionic bond.
An example of a polyatomic ion is sulfate (SO42−). When an ionic compound contains polyatomic ions, the same principles apply: the charges must balance to create a neutral compound. However, in the case of Rb and I, they are not polyatomic, so the basic rules of binary ionic compounds apply.