Answer: Rb₂S
Explanation:
Rubidium is in group one, so it has one outer electron, forming a 1+ ion when it becomes a rubidium ion.
Sulfur is in group six or sixteen, so it has six outer electrons, forming a 2- ion when it becomes a sulfide ion.
When these two elements bond it is an ionic bond, as it is between a metal and non-metal.
As you need to balance the charges for ionic compounds, there must be two rubidium ions so that the two rubidium ions (2+) and sulfide ion (2-) charges cancel out/are equal.
As a result, it is Rb₂S.
