I am studying solid state physics and I'm a complete newbie in that sense. I know that semiconductors and group IV elements bond themselves in the FCC structure with covalent bonds which satiate the requirements of the electronic configurations. What I don't understand is how metals like Aluminum or Calcium form crystal structures? How do the bonds satisfy in order to form a crystal structure. Or even, how does Phosphorous exist with other atoms of phosphorous to form crystal structures?


Categories of Solids Based on Bonds that Hold the Solid Together

Solids can be classified on the basis of the bonds that hold the atoms or molecules together. This approach categorizes solids as either molecular, covalent, ionic, or metallic.

Iodine (I2), sugar (C12H22O11), and polyethylene are examples of compounds that are molecular solids at room temperature. Water and bromine are liquids that form molecular solids when cooled slightly; H2O freezes at 0oC and Br2 freezes at -7oC.

Molecular solids are characterized by relatively strong intramolecular bonds between the atoms that form the molecules and much weaker intermolecular bonds between these molecules. Because the intermolecular bonds are relatively weak, molecular solids are often soft substances with low melting points.

Dry ice, or solid carbon dioxide, is a perfect example of a molecular solid. The van der Waals forces holding the CO2 molecules together are weak enough that dry ice sublimes--it passes directly from the solid to the gas phase--at -78oC.

diagram enter image description here

Covalent solids, such as diamond, form crystals that can be viewed as a single giant molecule made up of an almost endless number of covalent bonds. Each carbon atom in diamond is covalently bound to four other carbon atoms oriented toward the corners of a tetrahedron, as shown in the figure below. Because all of the bonds in this structure are equally strong, covalent solids are often very hard and they are notoriously difficult to melt. Diamond is the hardest natural substance and it melts at 3550C.

Diagram enter image description here

Ionic solids are salts, such as NaCl, that are held together by the strong force of attraction between ions of opposite charge.

Equation enter image description here

Because this force of attraction depends on the square of the distance between the positive and negative charges, the strength of an ionic bond depends on the radii of the ions that form the solid. As these ions become larger, the bond becomes weaker. But the ionic bond is still strong enough to ensure that salts have relatively high melting points and boiling points.

To understand metallic solids we have to clear up a common misconception about chemical bonds. Ionic and covalent bonds are often imagined as if they were opposite ends of a two-dimensional model of bonding in which compounds that contain polar bonds fall somewhere between these extremes.

ionic ........ polar ........ covalent

In reality, there are three kinds of bonds between adjacent atoms: ionic, covalent, and metallic, as shown in the figure below. The force of attraction between atoms in metals, such as copper and aluminum, or alloys, such as brass and bronze, are metallic bonds.

Diagramenter image description here

Molecular, ionic, and covalent solids all have one thing in common. With only rare exceptions, the electrons in these solids are localized. They either reside on one of the atoms or ions or they are shared by a pair of atoms or a small group of atoms.

Metal atoms don't have enough electrons to fill their valence shells by sharing electrons with their immediate neighbors. Electrons in the valence shell are therefore shared by many atoms, instead of just two. In effect, the valence electrons are delocalized over many metal atoms. Because these electrons aren't tightly bound to individual atoms, they are free to migrate through the metal. As a result, metals are good conductors of electricity. Electrons that enter the metal at one edge can displace other electrons to give rise to a net flow of electrons through the metal.

  • $\begingroup$ Is there a numerical parameter that checks and confirms how polar the bond is, I mean like it has to be probabilistic like a weighted variable, right? I mean there has to be some sort of parametric characterization to place the material in a given position in the above mentioned triangle. $\endgroup$ – ubuntu_noob Oct 25 '15 at 13:41

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