Answer:
When a compound chemical formula does not include any number it means that compound has only one atom of the elements the compound is made up of.
For example, HCN is a compound made up of one atom of hydrogen, carbon, and nitrogen so it does not contain any number in its chemical formula.
The number present in front of any chemical formula of a compound signifies that there are that many molecules of the element is present in the compound. For example, 3O₂ contains 3 molecules of oxygen not 3 atoms of oxygen.