The full-size number at the start of a chemical formula represents the number of molecules of that compound.

In your example, 3C2 means:

* 3: There are three molecules of the compound.

* C2: The compound itself consists of two carbon atoms (C).

However, the formula C2 is incomplete and doesn't represent a stable compound. It's likely that there's a missing element or subscript. For instance, it could be C2H6 (ethane) or C2H4 (ethylene).

Here are some other examples:

* 2H2O: Two molecules of water (H2O).

* 5NaCl: Five molecules of sodium chloride (NaCl).

* 4CO2: Four molecules of carbon dioxide (CO2).

Important Note: If the number is a subscript (small number below the element symbol), it represents the number of atoms of that element in a single molecule. For example, H₂O has two hydrogen atoms and one oxygen atom per molecule.