Difference Between Free Radical and Ion

The key difference between free radical and ion is that the free radicals have one or more unpaired electron, but ions have paired electrons.

We can explain the difference between free radical and ion from the basic properties of an ion and a free radical. An ion can occur as a molecule or atom with a charge (positive or negative) due to the loss or gain of an electron. Ions hold a negative charge due to a gain of an electron and hold a positive charge due to loss of an electron. Ions may occur as single or multi-charged chemical species, depending on the number of electrons gained or lost. Free radicals are molecules or atoms that have at least one unpaired electron. This article is about the difference between free radical and ion, including their special characteristics.

CONTENTS

1. Overview and Key Difference
2. What is Free Radical
3. What is Ion
4. Side by Side Comparison – Free Radical vs Ion in Tabular Form
5. Summary

What is Free Radical?

A Free radical is an atom or a group of atoms containing one or more unpaired electron(s). They are highly reactive due to the presence of an unpaired electron. Free radicals are very unstable and try to gain stability by accepting the needed electron. They react with other chemical compounds by capturing the needed electron. Free radicals are important intermediates in natural processes. We can denote free radicals by a superscript dot to the right. For example,  H., Cl., HO., H3C.

Figure 01: The Hydroxyl Radical

Long-lived free radicals are in three categories: stable radicals, persistent radicals, and di-radicals.

  1. Stable radicals: The major example of a stable radical is molecular oxygen O2. Organic radicals containing conjugated π system may long live.
  2. Persistent radicals: They are long-lived due to the steric crowding around the radical centre and make them physically difficult to react with another molecule.
  3. Di-radicals: Some molecules have two radical centres, we name them as di-radicals. Molecular oxygen naturally (atmospheric oxygen) exists as a diradical.

What is Ion?

Ions can form when a chemical species gains or loses electrons in chemical reactions; they have a positive (+) or negative (-) charge. Those get a negative charge by accepting an electron(s) and positive charge by donating electrons to an electron deficiency molecule or an element. Accepting or donating electrons directly affect the size of the ion; it changes the molecular size dramatically. We name an atom or a group of atoms without a negative or positive charge as “neutral”; to become a neutral atom or a molecule, the number of protons needs to be the same as a number of electrons.

Figure 02: Cation and Anion Formation

Therefore there are two forms of ions as follows.

  1. Cations or (+) ions – often metals comes under this category since metals lose electrons to become positive (+) charged (Na+, Ba2+, Ca2+, Al3+)
  2. Anions (-) ions – often nonmetals comes under this category since nonmetals gain electrons to become negative (-) charged (Cl–, S2-, O2-, Br–)

What is the Difference Between Free Radical and Ion?

The key difference between free radical and ion is that the free radicals have one or more unpaired electron, but ions have paired electrons. Therefore, free radicals are very unstable while ions are relatively stable. Hence, this is also a significant difference between free radical and ion. However, radicals can exist by themselves while most of the ions are combined with oppositely charged ions. When considering more about their stability, free radicals become stable by accepting electrons, but ions are stable when they form complexes with oppositely charged compounds.

Another important difference between free radical and ion is that the ions always hold a charge, but free radicals are not charged species even if they have unpaired electrons. This difference arises because, in an ion, the total number of electrons is always not equal to the number of protons in the nucleus while in a free radical, the number of electrons is equal to the number of protons.

The below infographic provides more details on the difference between free radical and ion.

Summary – Free Radical vs Ion

We can describe both the terms, free radicals and ions, using the number of electrons belonging to a particular species. Here, the most significant difference between free radicals and ions is that the free radicals have unpaired electrons but, ions have paired electrons. Thus, free radicals are more reactive. On the other hand, ions become chemically stable by forming compounds with oppositely charged ions/molecules.

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