Comparison between ionic, covalent, and metallic materials

Comparison between Ionic, Covalent, and Metallic Materials
- Practical Electron Microscopy and Database -
- An Online Book -

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Table 3231a. Comparison between ionic, covalent, and metallic materials.

Interatomic bond	Ionic	Covalent	Metallic
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	Higher	Higher	Intermediate
Mechanical property	Brittle, hard	Brittle, hard	Ductile, soft
Electrical property	Insulative	Insulative	Conductive
Example	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	Substance	Bonding energy			Melting point (°C)
Bonding type	Substance	kJ/mol	kcal/mol	eV/Atom, Ion, or Molecule	Melting point (°C)
Ionic	Typical value	50-1000
	NaCl	640	153	3.3	801
	MgO	1000	239	5.2	2800

Covalent	Typical value	200-1000
	Si	450	108	4.7	1410
	Diamond C	713	170	7.4	>3550

Metallic	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

Hydrogen	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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