--- title: "Primary and secondary bonds in solids" subject: "Material Science" chapter: 2 key: "semester_3/material_science:primary-and-secondary-bonds" source: https://www.bctengineer.com/q/semester_3/material_science/primary-and-secondary-bonds site: BCT Engineering --- # Primary and secondary bonds in solids Material Science · Chapter 2: Atomic Structure, arrangement of atoms - Asked in: year not given, 6 marks - Video: [Types of Bonding - Primary bonds & Secondary bonds](https://www.youtube.com/watch?v=eVdL-ipUa1Y) — Academic Gain Tutorials - Answer checked by Claude Opus 5.5 (high effort) on 2026-10-14 - Page: https://www.bctengineer.com/q/semester_3/material_science/primary-and-secondary-bonds - Whole chapter: https://www.bctengineer.com/md/semester_3/material_science/chapter-2 ## Question Explain the types of atomic bonds in solids. Sketch the force and potential-energy curves against interatomic distance and define bond (binding) energy. ## Short answer Primary bonds: ionic (electrostatic, non-directional), covalent (shared electrons, directional), metallic (electron cloud); they give high strength and melting points. Secondary bonds: van der Waals and hydrogen, weaker. At equilibrium spacing r0, net force is zero and potential energy is minimum. Bond energy Eb is the depth of this well, the energy needed to separate atoms. ## Answer Atoms bond to lower their energy. Bonds are **primary** (strong, 100-1000 kJ/mol) or **secondary** (weak, below about 40 kJ/mol). #### Primary bonds - **Ionic bond**: a metal atom transfers electrons to a non-metal atom, and the resulting opposite ions are held together by electrostatic attraction (NaCl, MgO). The bond is non-directional; ionic solids are hard, brittle, high melting and insulating in the solid state. - **Covalent bond**: atoms share electrons in a fixed direction (diamond, Si, SiC). Very hard, high melting, poor ductility. - **Metallic bond**: valence electrons form a free "electron cloud" shared by positive ion cores (Cu, Fe). Gives ductility, good electrical and thermal conductivity, and lustre. #### Secondary bonds - **Van der Waals** forces between temporary or permanent dipoles, and **hydrogen bonds** (H bonded to O, N, F). They are responsible for the bonding between polymer chains and in ice, and they explain the low melting point of polymers. #### Force and energy curves When two atoms are far apart they attract each other; when they come very close their electron clouds overlap and they repel. The net force is $F_N = F_A + F_R$. The attractive force dominates at large distance and the repulsive force at small distance. At the equilibrium spacing $r_0$ the net force is zero and the potential energy is minimum. **Force against interatomic distance:** ```text Force F ^ | attraction (+) _.-----.__ | .-' `--..___ 0 ----------------+------------------------> r | /| r0 (net force F = 0) | / | / net force F_N | repulsion (-)| | | ``` **Potential energy against interatomic distance:** ```text Energy E ^ | repulsion | | \ | \ 0 -------------\---------------------------> r | \ ___.....----- | \ _.-' | `-.__.' net energy E_N | | | -Eb (minimum) r0 ``` $$ E_N = -\frac{A}{r^m} + \frac{B}{r^n} \quad (n > m), \qquad \frac{dE_N}{dr} = 0 \ \ (\text{net force zero})\ \text{at}\ r = r_0 $$ **Bond (binding) energy** $E_b$ is the energy needed to separate two atoms from the equilibrium spacing $r_0$ to infinity, i.e., the depth of the energy well. Materials with deep, narrow wells have a high melting point, high modulus and low thermal expansion. ## Check yourself The answer's key points as multiple-choice questions, with the correct option and why. 1. What gives metals ductility and conductivity? A. Electron transfer to form ions B. A free electron cloud shared by positive ion cores C. Dipole attraction between chains D. Shared electrons in fixed directions Answer: B. The delocalised electron cloud in metallic bonding allows conduction and plastic flow. (Key point: Metallic bond) 2. Which bonds explain the low melting point of polymers? A. Ionic bonds within chains B. Covalent bonds along chains C. Van der Waals and hydrogen bonds between chains D. Metallic bonds Answer: C. Weak secondary bonds between chains are broken at low temperature. (Key point: Secondary bonds) 3. What is true at equilibrium spacing r0? A. Net force is zero and potential energy is minimum B. Repulsive force is zero C. Attractive force is zero D. Potential energy is zero Answer: A. Attraction and repulsion balance at r0, where the energy well is deepest. (Key point: Equilibrium spacing) 4. What is bond (binding) energy? A. The energy at infinite separation B. The force needed to hold atoms at r0 C. The energy needed to separate two atoms from r0 to infinity (depth of the well) D. The sum of attractive and repulsive forces Answer: C. Eb is the depth of the energy well, i.e. energy to separate atoms from r0 to infinity. 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