Ionic Bond

Atoms are the basic units of matter, but most atoms do not exist independently in nature. They combine with other atoms to form molecules and compounds. The force that holds atoms together in a molecule or compound is called a chemical bond. Chemical bonding occurs because atoms try to achieve a more stable electronic configuration, usually by completing their outermost shell (octet rule).

An ionic bond or electrovalent bond is a type of chemical bond formed when one atom completely transfers one or more electrons to another atom. This transfer of electrons usually happens between a metal and a non-metal.

Metals generally have 1, 2, or 3 valence electrons and tend to lose them easily to achieve a stable electronic configuration.
Non-metals usually have 5, 6, or 7 valence electrons and tend to gain electrons to complete their octet (8 electrons in the outer shell).
When a metal loses electron(s), it becomes a positively charged ion (cation).
When a non-metal gains electron(s), it becomes a negatively charged ion (anion).
The oppositely charged ions attract each other due to the strong electrostatic force of attraction, and this force forms an ionic bond.

Example: Formation of NaCl

Properties of Ionic Bond

Ionic bonds are formed by the complete transfer of electrons, resulting in positively and negatively charged ions. Because of the strong electrostatic attraction between these ions, ionic compounds show the following properties:

1. High Melting and Boiling Points

Ionic compounds have very high melting and boiling points.
There is strong electrostatic force of attraction between oppositely charged ions.
A large amount of energy is required to break these forces.

Example: NaCl melts at about 801°C.

2. Hard and Brittle Nature

Ionic compounds are hard but brittle.
Ions are arranged in a fixed crystal lattice.
When force is applied, similarly charged ions come close and repel each other, causing the crystal to break.

Example: Common salt (NaCl) crystals break easily when hit.

3. Solubility in Water

Most ionic compounds are soluble in water.
Water is a polar solvent and can separate the positive and negative ions.
However, ionic compounds are usually insoluble in non-polar solvents like petrol or benzene.

Example: NaCl dissolves in water.

4. Electrical Conductivity

Ionic compounds conduct electricity only in molten or aqueous state.
In solid state → Ions are fixed and cannot move.
In molten or dissolved state → Ions are free to move and carry charge.

Example: Molten NaCl conducts electricity.

5. Formation of Crystalline Solids

Ionic compounds form regular, well-defined crystal structures.
Ions are arranged in a repeating 3D pattern called crystal lattice.

6. Strong Electrostatic Forces

The bond between ions is strong due to electrostatic attraction between opposite charges.
Higher charge on ions → Stronger ionic bond.

Example:
MgO has stronger ionic bond than NaCl (because Mg²⁺ and O²⁻ have higher charges).

Formation of Ionic Bond

The formation of an ionic bond depends on several important factors that make electron transfer possible and stable.

1. Low Ionisation Energy

Ionisation energy is the energy required to remove an electron from an atom.
For ionic bond formation, the metal should have low ionisation energy so that it can lose electron easily.

Example:
Sodium (Na) easily loses one electron:
Na → Na⁺ + e⁻
Because Na has low ionisation energy, ionic bond formation becomes easier.

2. High Electron Affinity

Electron affinity is the energy released when an atom gains an electron.
The non-metal should have high electron affinity so that it can gain electron easily.

Example:
Chlorine (Cl) gains one electron:
Cl + e⁻ → Cl⁻
Chlorine has high electron affinity, so it readily forms Cl⁻ ion.

3. Large Difference in Electronegativity

The greater the difference in electronegativity between two atoms, the greater the chance of ionic bond formation.
Generally, electronegativity difference > 1.7 leads to ionic bond.

Example:
Na (low electronegativity), Cl (high electronegativity) → Forms ionic bond in NaCl.

4. High Lattice Energy

Lattice energy is the energy released when oppositely charged ions come together to form a solid crystal.
Higher lattice energy makes the ionic compound more stable.
Stronger attraction between ions → stronger ionic bond.

5. Stable Electronic Configuration (Octet Rule)

Atoms form ionic bonds to achieve stable noble gas configuration (octet).

Example:
Na (2,8,1) → becomes 2,8 after losing 1 electron.
Cl (2,8,7) → becomes 2,8,8 after gaining 1 electron.
Both become stable.

Preparation of Ionic Bond

An ionic bond is formed when one atom completely transfers one or more electrons to another atom. This usually happens between a metal and a non-metal.

The formation of an ionic bond takes place in three main steps:

Step 1: Formation of Positive Ion (Cation)

The metal atom loses one or more electrons to become a positively charged ion.

Metals have:

Low ionisation energy
1 or 2 valence electrons
Tendency to lose electrons

Step 2: Formation of Negative Ion (Anion)

The non-metal atom gains the electron lost by the metal and becomes negatively charged.

Non-metals have:

High electron affinity
5, 6, or 7 valence electrons
Tendency to gain electrons

Step 3: Formation of Ionic Bond

Now the oppositely charged ions attract each other due to strong electrostatic force. This attraction forms the ionic bond.

Formation of NaCl

In NaCl, sodium (Na), a metal, loses one electron to achieve a stable noble gas configuration, while chlorine (Cl), a non-metal, gains that electron to complete its octet. The resulting oppositely charged ions (Na⁺ and Cl⁻) are held together by strong electrostatic forces of attraction, forming a stable ionic lattice.

a) Electronic Configuration of Atoms

Sodium (Na), Atomic number = 11
Electronic configuration = 2, 8, 1

Chlorine (Cl), Atomic number = 17
Electronic configuration = 2, 8, 7

Sodium has 1 valence electron.
Chlorine has 7 valence electrons.

Both want to achieve stable noble gas configuration (octet).

b) Loss of Electron by Sodium

Sodium easily loses one electron because it has low ionisation energy.

Na → Na⁺ + e⁻

After losing one electron:
Na⁺ configuration = 2, 8

Now sodium becomes stable like Neon.

c) Gain of Electron by Chlorine

Chlorine needs one electron to complete its octet.

Cl + e⁻ → Cl⁻

After gaining one electron:
Cl⁻ configuration = 2, 8, 8

Now chlorine becomes stable like Argon.

d) Formation of Ionic Bond

Now Na⁺ and Cl⁻ are oppositely charged ions.

Na⁺ (+ charge)
Cl⁻ (– charge)

They attract each other due to strong electrostatic force.

Na⁺ + Cl ^- → NaCl

This attraction forms the ionic bond.

Examples of Ionic Bonds

There are various compounds which are formed by ionic bonds and these compounds are called ionic compounds. Various examples of the ionic bond are discussed in the table below,

Compound	Cation	Anion	Reaction	Electronic Configuration
NaCl	Na⁺	Cl^-	Na → Na⁺ + e^– Cl + e^–→ Cl^–	Na = 2, 8, 1 Cl = 2, 8, 7
CaCl₂	Ca²⁺	2Cl^-	Ca → Ca²⁺ + 2e^– 2Cl + 2e^–→ 2Cl^–	Ca = 2, 8, 8, 2 Cl = 2, 8, 7
CaO	Ca²⁺	O^2-	Ca → Na²⁺ + 2e^– O + 2e^– → O^2-	Ca = 2, 8, 8, 2 O = 2, 6

Ionic Bond vs Covalent Bond

Ionic Bond	Covalent Bond
Ionic bonds are formed by the attraction of positive and negative ions in a crystal, and compounds held together by ionic bonds are known as ionic compounds.	When two atoms share one or more electron pairs, they form a covalent bond. Each atom contributes an equal number of electrons to the formation of the bond.
In the formation of ionic bonds cations and anions are formed and held together by electrostatic attraction.	No such intermediates are formed in the formation of covalent bonds.
Ionic bonds are formed between two atoms in which one atom is electronegative and the other is electropositive.	As atoms with higher ionization potential do not lose their valence electrons easily, they prefer to form covalent bonds by sharing electrons.
If the electronegativity difference between the atom is very high they form ionic bonds. For instance, bonds formed in the KCl compound.	If the electronegativities of the combining atoms are not significantly different, the bond formed between them is most likely covalent. For instance, bonds formed in the HCl compound
Examples of the compound with ionic bonds are, NaCl CaCl₂ KOH, etc.	Examples of the compound with covalent bonds are, H₂O NH₃ CO₂, etc.

Types of Chemical Bonds
Bond Parameters