Electron microscopy
Comparison between Ionic, Covalent, and Metallic Materials
- Practical Electron Microscopy and Database -
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Table 3231a. Comparison between ionic, covalent, and metallic materials.

Interatomic bond


Formation mechanism
Mutual ionization occurs by electron transfer; Results from the attraction of oppositely charged ions; Ions are attracted by strong coulombic interaction Is created when two atoms share a pair of electrons. All the atoms are ionized, loosing some electrons from the valence band and thus the valence electrons are shared collectively by all the atoms.
Electronegativity difference

> ~2

Electronegativity difference: < ~2

Small in an alloy
Valence electron sharing

Very little sharing, so the bonds are non-directional (ions may be attracted
to one another in any direction; delocalized)

High-degree sharing, so the bonds are very directional (localized) Is averaged over many metal atoms rather than just two atoms, resulting in an electron gas.
Charge on atoms
Positively or negatively charged No net charge on either atom No net charge on any atoms
Melting point



Mechanical property

Brittle, hard

Brittle, hard

Ductile, soft

Electrical property

Insulative Insulative Conductive
Na+Cl- H2 W

As listed in Table 3231b, substances with large bonding energies usually have high melting temperatures.

Table 3231b. Bonding energies and melting temperatures of different substances*.

Bonding type

Bonding energy
Melting point (°C)
kJ/mol kcal/mol eV/Atom, Ion, or Molecule
Typical value 50-1000      
NaCl 640 153 3.3 801
MgO 1000 239 5.2 2800
Typical value 200-1000      
Si 450 108 4.7 1410
Diamond C 713 170 7.4 >3550
Typical value 50-1000      
Hg 68 16 0.7 -39
Al 324 77 3.4 660
Fe 406 97 4.2 1538
W 849 203 8.8 3410
van der Waals
Typical value 0.1-10      
Ar 7.7 1.8 0.08 -189
Cl2 31 7.4 0.32 -101
Typical value 10-40      
NH3 35 8.4 0.36 -78
H2O 51 12.2 0.52 0

           * The one which has the largest bonding energy and highest melting point in the same bond-type of substances is colored in red.



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