Interatomic and Intermolecular Forces Explained | Class 11 Physics Notes

 

Interatomic and Intermolecular Forces

The physical world that we see around us — solids, liquids, and gases — is held together by forces of attraction at the atomic and molecular levels. These forces determine whether matter exists as a crystal, a liquid droplet, or a free-flowing gas. In physics, we broadly divide these interactions into interatomic forces and intermolecular forces.

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1. Interatomic Forces (Forces inside a molecule or solid)

Interatomic forces are strong forces responsible for the formation of chemical bonds between atoms. They decide the internal structure and stability of matter.

(a) Ionic Bond

• Formed by transfer of electrons from one atom to another.

• Example: NaCl, where sodium loses an electron and chlorine gains it, creating positive and negative ions.

• They are held together by strong electrostatic attraction.

(b) Covalent Bond

• Formed by sharing of electrons between atoms.

• Example: H₂, O₂, H₂O.

• Very strong and directional in nature.

(c) Metallic Bond

• Present in metals.

• Positive ions are arranged in a lattice surrounded by a "sea of free electrons."

• Explains metallic properties like conductivity, ductility, and malleability.

Thus, interatomic forces are very strong and usually of the order of 10⁻⁹ N or more. They give matter its basic structure.

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2. Intermolecular Forces (Forces between molecules)

While interatomic forces bind atoms inside a molecule, intermolecular forces act between neighbouring molecules. They are comparatively weaker but control many physical properties such as melting point, boiling point, viscosity, and solubility.

(a) Van der Waals Forces

These are weak forces due to electrical interactions between molecules. They are of three types:

• London Dispersion Forces → Temporary dipoles due to movement of electrons (present even in noble gases like He, Ar).

• Dipole–Dipole Forces → Attraction between molecules having permanent dipoles (e.g., HCl).

• Dipole–Induced Dipole Forces → A polar molecule induces a temporary dipole in a nonpolar molecule.

(b) Hydrogen Bonding

• A special type of dipole interaction.

• Formed when a hydrogen atom is covalently bonded to a highly electronegative atom (O, N, F) and is attracted to another electronegative atom.

• Example: In water (H₂O), hydrogen bonds explain its high boiling point and unusual density behaviour (ice floats on water).

• Essential for life processes (DNA structure, protein folding).

(c) Ion–Dipole Interaction

• Important in solutions.

• Example: Na⁺ and Cl⁻ ions surrounded by water molecules when NaCl dissolves in water.

Thus, intermolecular forces are weaker than interatomic forces, but they are very important for determining the physical state of matter.

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3. Comparison

Feature	Interatomic Forces	Intermolecular Forces
Strength	Very strong (bond energy 100–1000 kJ/mol)	Weaker (1–50 kJ/mol)
Role	Form molecules/solids	Decide physical properties
Examples	Ionic, covalent, metallic bonds	Van der Waals, hydrogen bonding

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4. Importance in Physics

• Explain why matter exists in solid, liquid, or gaseous states.

• Explain anomalies of water (ice less dense than water).

• Responsible for elasticity, viscosity, surface tension, and solubility.

• Crucial in modern applications like nanotechnology, semiconductors, and biological systems.

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Conclusion

The universe at the microscopic level is governed by invisible interactions. Interatomic forces build the basic structure of matter, while intermolecular forces govern the behaviour of substances in bulk. Together, they explain why solids are rigid, liquids flow, gases expand, and why life itself is possible. Understanding these forces is therefore a cornerstone of Class XI Physics and a bridge between chemistry and physics.