So far, we learned about ionic solids, metallic solids, and alloys
. Here's some more you should know!
The structures and properties of solids can be classified according to the forces that hold the atoms together.
Ionic solids are held together by the mutual attraction between cations and anions.
Covalent network solids are held together by an extended network of covalent bonds (Think of diamonds💎).
Molecular solids are held together by weak IMFs. Now that the basics are out of the way, let’s dig into the details!
These are their own category and include solids like gum and glass. In amorphous solids, particles show no long-range order. They have considerable disorders in their structures and it is all due to cooling quickly😰.
Crystalline solids are the greater category of solids and include all of the different types of solids below.
In crystalline solids, particles are arranged in a regularly repeating pattern. The smallest repeating unit is called a unit cell and the geometrical pattern of points on which the unit cells are arranged is called a crystal lattice.
💡Quick Analogy🧊: If one cube represented a unit cell, then 100 cubes combined into a structure would represent a crystal lattice.
Image Courtesy of OpenTextBC
Metallic solids are typically good conductors of electricity and heat, malleable (can be hammered into thin sheets), and ductile (can be drawn into wires). The properties of metals can be accounted for in a qualitative way by the electron-sea-model, in which the valence electrons are delocalized and are visualized as being free to move throughout the metal.
Animation Courtesy of DocSity
Alloys are materials that possess characteristic metallic properties and are composed of more than one element. The elements in an alloy can be distributed either homogeneous or heterogeneous.
In a substitutional alloy, the atoms of the minority element occupy positions normally occupied by atoms of the majority element.
In an interstitial alloy, atoms of the minority elements, often smaller nonmetallic atoms, occupy interstitial positions that lie in the “holes” between atoms of the majority element.
Image Courtesy of the University of North Florida
Ionic solids consist of cations and anions held together by electrostatic attractions. Since these interactions are quite strong, ionic compounds tend to have a higher melting point.
💡The key trend to note is that the attractions become stronger as the charges of ions increase and/or the sizes of the ions decrease (Coulomb's Law anyone?).
The presence of both attractive and repulsive interactions helps to explain why ionic compounds are brittle. Ionic solids are characterized by a crystal lattice of ions attracted to each other.
They only conduct electricity when ions are mobile and can flow. This occurs when ionic solids are melted or put into solution.
Something new to note about ionic solids is that they possess ions at the lattice points of the solid structure.
Molecular solids consist of atoms or molecules held together by IMFs. Because these IMF forces are relatively weak, molecular solids tend to be soft and possess low melting points. The melting point depends on the strength of the IMFs, as well as the efficiency with which the molecules can pack together. A good example of a molecular solid is ice and sucrose.
Remember, they possess strong intramolecular forces (covalent bonds), but weak intermolecular forces.
They cannot conduct electricity because their valence electrons are tightly held within the covalent bonds, making them unable to move.
Covalent Network Solids
Covalent network solids consist of atoms held together in large networks by covalent bonds. These solids are much harder and have higher melting points than molecular solids. Important examples to keep in mind are graphite and diamond.
These large networks could be atoms bonded in a 3D network, like diamonds, or 2D layers, like graphite.
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Here are some key facts about both:
Layers of Carbon atoms arranged in rings
sp2 hybridized, pi bonds
delocalized electrons --> good conductor
strong bonds within the layers, but weak bonds between the layers
soft because the layers can slide past each other
When comparing properties among the different solids, remember this chart:
|Type of Solid||Form of Unit Particles||Forces Between Particles||Properties||Examples|
|Molecular🧊||Atoms or Molecules||LDFs, dipole-dipole, hydrogen bonding||fairly soft, low melting point, bad conductor||Argon, methane, sucrose, dry ice|
|Covalent Network💎||Atoms connected in a network of covalent bonds||Covalent Bonds||Very hard, very high melting point, bad conductor||diamond, quartz|
|Ionic🧂||Positive and Negative Ions||Electrostatic attractions||Hard and brittle, high melting point, bad conductor ||salts (NaCl)|
|Metallic✨||Atoms||Metallic Bonds||Varying hardness and melting points, good conductor, malleable, ductile||metals! Cu, Fe, Al|
Table Courtesy of Unknown Source