Cations and Anions: According to its atomic number, every atom has the same number of protons and electrons in its ground state, i.e. neutral or uncharged. However, an atom often gains or loses electrons depending on its electronegativity. The force that drives such gain or loss of electrons is the urge of atoms to possess octet configuration, i.e., having a complete valence shell of electrons. The resulting charged atom has the electronic configuration of a noble gas. The addition of an electron will disturb the proton-electron balance and leave the atom negatively charged. Similarly, the removal of an electron will leave the atom positively charged. Let’s explore what these charged species are.

Ions

An atom comprises a dense nucleus which is made up of positively charged protons and neutral neutrons surrounded by negatively charged electrons. An atom is electronically neutral if it has the same number of protons and electrons. However, if the total number of protons does not equal the number of electrons, the atom possesses a net electrical charge.

Any atom or molecule with a net electrical charge is known as an ion. The charge on the ion can be either positive or negative, depending on whether there is a loss or gain of electrons. A monoatomic ion is an ion consisting of a single atom is; whereas a polyatomic ion consists of two or more atoms.

Types of Ions

There are mainly three types of ions. These are:

1. Anions 
2. Cations 
3. Zwitterions

Cation

A cation is an atom or a group of atoms with more protons than electrons, consequently giving it a net positive charge.

Formation of cation

Cations are formed-

1. When there is a removal or loss of electrons from neutral atoms or other molecules.
2. By the combination of positive ions with other molecules.
3. By the cleavage of the covalent bond causes the shared pair of electrons to be associated with one atom constituting the covalent bond. The other atom becomes an electron deficit resulting in a cation.

Metal atoms usually form cations.
These are electron-deficient species; hence they possess a net positive charge. Cations are generally smaller than their parent atom or molecule due to fewer electrons that result in the smaller size of their electron clouds. When subjected to an electric field, cations move towards the negative terminal (cathode) to form neutral atoms. 

The positively charged ions of organic molecules are known as carbocations.

Anion

An anion is an atom, or a group of atoms with more electrons than protons, consequently giving it a net negative charge.

Formation of anion

Anions are formed-

1. When there is an addition or gain of electrons to neutral atoms or other molecules.
2. By the combination of negative ions with other molecules.
3. By the cleavage of a covalent bond causes the shared pair of electrons to be associated with one atom resulting in a negative charge.

Non-metals usually form anions.

These are electron-rich species; hence they possess a net negative charge. Anions are generally bigger than their parent atom or molecule because the excess electrons suffer repulsion and contribute to the physical size of the electron cloud.
Anions move towards the positive terminal (anode) to form neutral atoms when subjected to an electric field. 

Negatively charged ions formed from organic molecules are termed carbanions.

Zwitter Ions

These ions are neutral and possess both positive and negative charges at different locations throughout the molecule. Amino acids are the most common example of zwitterions. They are made up of an ammonium or amino group that contains a positive charge and a carboxyl group that contains a negative charge.

Ionisation energy

Cations or anions can be formed by ionisation, which is the process of a neutral atom gaining or losing electrons. Generally, the electrons are either lost or added to the outermost shell of an atom to achieve a stable electronic configuration that satisfies the octet rule.

Example of Cation

A common example of cation is \({\rm{N}}{{\rm{a}}^{\rm{ + }}}\) A sodium atom has eleven electrons. However, as per the octet rule, the sodium atom would be more stable with \(10\) electrons (\(2\) in its innermost shell, \(8\) in its outermost shell). Hence, sodium tends to lose an electron to become more stable and acquires \({\rm{a + 1}}\) charge. One electron is lost, hence \(+1\) charge. The ionisation of sodium is illustrated as follows:

\({\rm{Na}} \to {\rm{N}}{{\rm{a}}^{\rm{ + }}}{\rm{ + }}{{\rm{e}}^{\rm{ – }}}\)

The sodium atom could also gain seven electrons more to achieve a full valence shell. But removing one electron is much easier than gaining seven, and thus sodium achieves its octet by becoming a cation.

Example of Anion

A common example of anion is \({\rm{C}}{{\rm{l}}^{\rm{ – }}}.\) The valence shell of a chlorine atom consists of seven electrons. Hence, it tends to gain an electron to acquire the stable octet configuration and becomes \({\rm{C}}{{\rm{l}}^{\rm{ – }}}.\) However, as per the octet rule, the chlorine atom would be more stable with \(18\) electrons (\(2\) in its innermost shell, \(8\) in its second shell, and \(8\) in its valence shell). Hence, the chlorine atom will take an electron from another atom to become more stable and acquire \({\rm{a – 1}}\) charge. One electron is gained, hence \({\rm{a – 1}}\) charge.

\({\rm{Cl}} + {{\rm{e}}^ – } \to {\rm{C}}{{\rm{l}}^ – }\)

Formation of Ionic Compounds

Combining the propensity of sodium to lose an electron and of chlorine to gain an electron, we observe that when combined, the uncharged atoms can exchange electrons, and by doing so, they can achieve complete valence shells. These oppositely charged attract one other to form ionic bonds and produce ionic compounds with no overall net charge. The reaction is as follows:

\({\rm{N}}{{\rm{a}}^ + } + {\rm{C}}{{\rm{l}}^ – } \to {\rm{NaCl}}\)

Chemical Notation of Cations and Anions

The chemical formula for an ion is written with its net charge followed by its sign in superscript immediately after the chemical structure for the molecule or atom. Hence, a dication or a doubly charged cation is indicated as \(2+\) instead of \(+2.\) However, for singly charged molecules or atoms, the magnitude of the charge is omitted; for example, the sodium cation is indicated as \({\rm{N}}{{\rm{a}}^ + }\) and not \({\rm{N}}{{\rm{a}}^{1 + }}.\)

The ions of transition metals with multiple charges are shown by drawing out the signs multiple times. For example, a twice-positively charged iron atom can also be expressed as \({\rm{F}}{{\rm{e}}^{{\rm{2 + }}}}\) or \({\rm{F}}{{\rm{e}}^{{\rm{ + + }}}}{\rm{.}}\)

Cation vs Anion Periodic Table

Based on its position on the periodic table, it is possible to predict whether an atom will form a cation or an anion. As we move from left to right across the period, the tendency of an atom to form cations decreases—however, the tendency of an atom to form anions increases. Hence, halogens that belong to the extreme right of the periodic table always form anions, but alkali metals and alkaline earth metals that belong to the extreme left of the periodic table always form cations. However, some elements are capable of forming both cations and anions, given the right conditions. For example- hydrogen atom, which may gain \(\left( {{{\rm{H}}^{\rm{ – }}}} \right)\) or lose \(\left( {{{\rm{H}}^ + }} \right)\) an electron, forming hydride compounds such as \({\rm{Zn}}{{\rm{H}}_{\rm{2}}}\) (where it is an anion) and hydron compounds such as \({{\rm{H}}_{\rm{2}}}{\rm{O}}\) (where it is a cation).

Noble gases that belong to group \(18\) of the periodic table tend not to form ions. They have a stable octet configuration in their valence shells.

Cation vs Anion Size

A comparison of ionic radii with atomic radii shows that an anion, having gained an electron, is always larger than the parent neutral atom. Whereas a cation, having lost an electron, is always smaller than its parent neutral atom. 

When a cation is formed, two things happen: 

(1) A decrease in the repulsions between electrons in the same principal shell due to fewer electrons, and 
(2) An increase in the effective nuclear charge is felt by the remaining electrons as fewer electrons shield each other from the nucleus. Consequently, the size of the region of space occupied by electrons decreases and the ion shrinks. 

The cation with the greatest positive charge is the smallest in size.

When an anion is formed-

(1) An increase in the repulsions between electrons in the same principal shell occurs due to the addition of electrons, and 
(2) A decrease in the effective nuclear charge occurs as more electrons are present to shield each other from the nucleus. Consequently, the size of the region of space occupied by electrons increases and the ion expands. 

For example, the Na⁺ ion is significantly smaller than the neutral \({\rm{Na}}\left( {{\rm{1}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{s}}^{\rm{2}}}{\rm{2}}{{\rm{p}}^{\rm{6}}}{\rm{3}}{{\rm{s}}^{\rm{2}}}} \right)\)
atom. This is because in \({\rm{N}}{{\rm{a}}^{\rm{ + }}},\) an electron from the \({\rm{3}}{{\rm{s}}^{\rm{1}}}\) shell has been removed to give a closed shell with \({\rm{n = 2}}{\rm{.}}\) However, in \({\rm{N}}{{\rm{a}}^ – }\) an addition of an electron produces a \({\rm{3}}{{\rm{s}}^2}\) valence electron configuration. The electronic repulsion increases and the \({\rm{N}}{{\rm{a}}^ – }\) ion is larger than the parent Na atom.

Ionic radii of ions with the same charge increases going down a column. This is because shielding by filled inner shells produces little change in the effective nuclear charge felt by the outermost electrons. The change in principal shells \(\left( {\rm{n}} \right)\) places electrons at successively greater distances from the nucleus.

Key Differences (Cation vs Anion)

Basis for Comparison	Cation	Anion
Definition	It is an atom or a group of atoms bearing one or more positive electric charges.	It is an atom or a group of atoms bearing one or more negative electric charges.
Electric charge	Positive charges	Negative charges
Atoms	Metals form cations	Non-metals form anions
Electric field	Cations are attracted towards the negative terminal (cathode) of an electric field.	Anions are attracted towards the positive terminal (anode) of an electric field.
Reactions	Cations react with anions to form neutral molecules.	Anions react with cations to form neutral molecules.
Electrons	Cations have more protons than electrons.	Anions have more electrons than protons.
Size	Cations are smaller in diameter than their parent atom.	Anions are larger in size than their parent atom.
Organic ions	Organic cations are known as carbocations.	Organic anions are known as carbanions.
Crystal lattice	These occupy space between two anions (interstitial space) in the crystal lattice.	These occupy most of the space in the crystal lattice.
Examples	\({\rm{N}}{{\rm{a}}^ + },{{\rm{K}}^ + },{\rm{NH}}_4^ + ,{\rm{C}}{{\rm{a}}^{2 + }}\) and \({\rm{A}}{{\rm{l}}^{3 + }}.\)	\({\rm{SO}}_4^ – ,{\rm{C}}{{\rm{l}}^ – },{{\rm{F}}^ – },{\rm{PO}}_4^ – .\)

Summary

When one or more electrons are removed from a neutral atom, a positive ion (cation) is formed. When one or more electrons attach themselves to neutral atoms, a negative one (anion) is formed. The designations cation or anion come from the early experiments with electricity in which positively charged particles were attracted to the cathode (negative terminal of a battery). In contrast, negatively charged ones were attracted to the anode (positive terminal of the battery).

Frequently Asked Questions (FAQs) on Cations and Anions

Q.1. Between anions or cations, which one is bigger?
Ans: Anions are larger, whereas cations are smaller than their corresponding neutral atom. This is because the addition of electrons increases the electron-electron repulsion, increasing the electron cloud. Whereas, removal of electrons results in decreased repulsion between electrons. Hence, anions are bigger, and cations are smaller than their corresponding parent atom.  

Q.2. Which is the smallest cation?
Ans: The hydrogen ion is the smallest cation. This is because hydrogen contains one electron; when it forms H+ ion, the only electron present in its valence shell is lost, and the atom is left with no electron in its shell. Hence, H+ ion is the smallest cation.

Q.3. How do you know if an ion is a cation or anion?
Ans: Suppose the chemical formula for an ion is written with its net charge followed by a positive \((+)\) sign in superscript immediately after the chemical structure for the molecule or atom. In that case, the ion is a cation.
Suppose the chemical formula for an ion is written with its net charge followed by a negative \((-)\) sign in superscript immediately after the chemical structure for the molecule or atom. In that case, the ion is an anion.

Q.4. What is anion exchange resin?
Ans: An anion exchange resin is an ion-exchange resin that contains basic chemical groups for exchanging anions.

Q.5. Is oxygen a cation or anion?
Ans: Oxygen belongs to group \(16\) of the periodic table and has \(6\) valence electrons. Hence, it can easily accept two electrons to satisfy the octet configuration. Hence, oxygen is an anion with magnitude \(2\) chemically represented as \({{\rm{O}}^{{\rm{2 – }}}}{\rm{.}}\)

Learn About Metallic Bonding Here

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