CHEMISTRY / SCIENCE

Difference between Ions and Radicals with Examples

by Richa Rastogi · Published June 27, 2023 · Updated June 28, 2023

Most people around the world consider ions and radicals the same, but there is a difference between the two. Yes, you read it right; there is a difference between ions and radicals.

An ion is an atom of an element or a group of atoms of different elements that behave as a single unit with a positive and negative charge on it. Whereas, radical (often called as ‘free radical’) is an atom of an element or a group of atoms of different elements that has at least one unpaired electron. Radicals are neutral, positive, and negative in charge.

Unlike ions, radicals have an odd number of electrons. Due to which, one electron is unpaired. This makes radicals, highly chemically reactive.

Difference between ions and radicals

IONS

The molecule of a compound usually salts in solution states splits into two parts with a positive and negative charge on them. These are ions. The positive ion is called a cation and the negative ion, an anion. For example, a molecule of sodium carbonate (Na₂CO₃) has two parts, sodium (Na⁺), and carbonate (CO₃^2-). Here, sodium is a positive ion and carbonate is a negative ion.

Ions bear positive or negative charges due to the loss or gain of an electron. An atom or a molecule is neutral as the number of protons present in it is equal to that of electrons. So, anytime atoms or molecules lose electrons, it results in the formation of positively charged ions i.e. cations. Similarly, if atoms or molecules gain extra electrons, it results in negatively charged ions i.e. anions. Therefore, the number of protons in ions is never equal to the number of electrons. If protons are more, an ion is positive and if protons are less, then ion is negative.

Therefore, from the above, the definition of ion is–

An ion is an atom of an element or a group of atoms of different elements that behave as a single unit with a positive and negative charge on it.

Other than positive and negative, we can also classify ion as –

Simple and compound ions
Acidic and basic ions

Simple Ions and Compound Ions:

The ion is simple if it consists of a single atom, Na¹⁺, Mg²⁺, Cl^1-, etc. Whereas compound ions are those which consist of more than one atom of different elements. For example, NH₄¹⁺, SO₄^2-, CO₃^2-, etc. Simple ions are monoatomic whereas compound ions are diatomic or polyatomic.

Acidic and Basic Ions:

When salt in the solution state, splits into two parts, the positive part is called a basic ion and the negative part an acidic ion.

Let’s, understand with an example, of a neutralization reaction between potassium hydroxide (a base) and hydrochloric acid. Here, the acid reacts with the base to form a salt (potassium chloride) and water. During the formation of this salt, the base potassium hydroxide contributes to potassium ion (K⁺). Therefore, (K⁺) is a basic ion. While hydrochloric acid contributes to chloride ions (Cl^–). Therefore, (Cl^–) is an acidic ion.

It is important to note here, that some people called “simple and compound ions” as “simple and compound radicals”. Similarly, “acidic and basic ions” as “acidic and basic radicals”. There is a reason for that and we will discuss it after studying about radicals.

RADICALS

All atoms except that of inert gases are chemically unstable. They try to stabilize by combining with other atoms of the same or different elements using the octet or duplet rule. However, sometimes even after combining with other atoms, the molecule formed is left with an unpaired electron in its valence shell. Such molecules with at least one unpaired electron in their outermost shell or valence shell are called radicals. Due to the presence of this unpaired electron, radicals are highly chemically reactive.

Electrons in atoms occur in the form of pairs, both rotating in an opposite spin. This opposite spin of electrons somewhat stabilizes the atom. However, atoms with an odd number of electrons always have one electron unpaired. Therefore, due to the absence of counter spin, these atoms are more unstable than those which have an even number of electrons. It is important to note here, that radicals always have an odd number of electrons, due to which always one electron is unpaired.

Mostly, the number of protons and the number of electrons are equal in radicals. Therefore, they are electrically neutral. However, this is not always true. Sometimes, radicals bear positive and negative charges due to the absence or presence of excess electrons. This is the reason why people mistake, “positive and negative ions” with “positive and negative radicals”. The radicals with charge are also known as radical ions and are mostly found in organic compounds.

Therefore, from the above, the definition of radical is–

Radical or ‘free radical’ is an atom of an element or a group of atoms of different elements that have at least one unpaired electron. Radicals are neutral, positive, and negative in charge.

Examples of radicals-

Elemental nitrogen and hydroxyl are the best examples of simple free radicals.

Nitrogen: Atomic number of nitrogen is 7. It means the nitrogen atom has 7 protons and 7 electrons. Due to the presence of an odd number of electrons, one electron is always left unpaired. Therefore, the nitrogen atom in a free state is considered radical. Elemental nitrogen radical is highly reactive and never exists in the free state in nature.

Hydroxyl: Refer difference between ions and radicals, below.

DIFFERENCE BETWEEN IONS AND RADICALS

To understand the difference between ions and radicals, let us consider an example of hydroxyl (OH). Hydroxyl ions and hydroxyl radicals are two different species, commonly mistaken as the same. A hydroxyl ion bears a negative charge, whereas a hydroxyl radical is neutral.

Hydroxyl radicals are the result of the homolytic fission of water molecules, whereas, the heterolytic fission results in hydroxyl ions. So, before we learn more about hydroxyl ions and hydroxyl radicals, let’s understand about homolytic and heterolytic fission.

Homolytic and Hetrolytic Fission

‘Fission’ means ‘to split’. During homolytic fission, a molecule of a compound breaks in such a manner that the splitting atoms retain one of the originally bonded electrons. Whereas, in heterolytic fission, one of the atoms moves away with both of the bonded electrons. This results in one set of fragments with more number of electrons than the others. The fragment with more electrons bears a negative charge and the one with fewer electrons bears a positive charge.

Homolytic and heterolytic fission of water molecules to form hydroxyl radicals and hydroxyl ions, respectively

Now let’s get back to hydroxyl ion and hydroxyl radical. During the homolytic fission of a water molecule, it splits into two fragments, hydroxyl (OH) and hydrogen (H). In this fission, the number of protons and the number of electrons in the hydroxyl (OH) fragment is the same i.e. nine. Therefore, it bears a neutral charge. However, as one unpaired electron exists in this hydroxyl fragment due to an odd number of electrons, thus it is a hydroxyl radical. Whereas, during heterolytic fission, the water molecule splits in such a way that the hydroxyl fragment (OH) moves away with both of the bonded electrons. This results in more electrons in comparison to that of protons. It has nine protons and ten electrons. Due to this excess electron, this hydroxyl fragment bears a negative charge and is thus called a hydroxyl ion.

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