To convert between moles and particles, Avogadro's number is essential. This constant, valued at approximately $6.02210^23$, indicates that one mole of any substance contains this many particles. The term "particles" is a broad term encompassing ions, atoms, molecules, or formula units, allowing for a unified approach to counting different types of chemical entities.

For example, one mole of chlorine gas (Cl₂), which is a diatomic molecule, corresponds to $6.02210^23$ molecules of Cl₂. This relationship can be expressed as a conversion factor: 1 mole of Cl₂ equals $6.02210^23$ molecules of Cl₂. This conversion factor can be inverted, allowing for flexibility in calculations depending on the desired outcome.

It's important to note why we refer to chlorine as molecules in this context. The term "molecules" applies when discussing compounds formed from two or more nonmetals, as is the case with Cl₂. In contrast, if we were discussing a single chlorine atom, we would use the term "atom." The term "ion" would not apply here since Cl₂ does not carry a charge, and "formula unit" is reserved for compounds that include both metals and nonmetals.

In summary, when converting between moles and particles, Avogadro's number serves as a crucial tool, particularly when dealing with molecular compounds like chlorine gas, where the appropriate terminology reflects the nature of the substance involved.