What is the formula for rubidium nitride ?

Hint: The molecular formula is a formula which consists of the chemical symbols for the elements the molecule is made up of following the numeric subscript which describes the number of atoms of that element.

Complete step by step answer:
Rubidium is a group one element having a valency $1$. And to attain noble gas configuration rubidium donates one electron from its outer shell and forms a cation. On the other hand nitride needs to gain three electrons to attain stability to form an anion.
Rubidium nitride is an ionic binary compound and so we add ide suffix after the anion. Nitrogen > nitride.
We can find the molecular formula in the following way:
Step I: We should first know the symbol of the atoms used. In this it is Rubidium that is Rb and nitride N .
Step II: Write the symbols below their names as shown
$\text{Rubidium Nitride}$
$\text{Rb N}$
Step III: Write the valency of the atom below as shown
$\text{Rubidium Nitride}$
$\text{Rb N}$
$\text{1 3}$
Step IV: Now cross multiply the valency here we will get $R{{b}_{3}}N$ .

Note: The molecular formula, atomic masses of each atom and other required things can be given and can also be calculated. Also, it is advised to know the atomic mass of most of the elements and most importantly of those which are most common.
Rubidium nitrite is often confused with rubidium nitrite having the formula $RbN{{O}_{2}}$ . Make sure to read the question properly and not make any mistakes in this.