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Chemical Bonding

PostPosted: April 26th, 2011, 4:02 pm
by Jessica
Ionic solids - Example: NaCl(s)

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Ionic solids such as NaCl(s) and CaI[sub]2[/sub](s) have relatively strong attractions between the ions

Their physical properties are as follows:
  1. Because there are no free moving electrons or ions, ionic solids do not conduct heat and electricity unless the ionic solid is dissolved (aqueous), or melted (liquid).
  2. Because particles cannot slide over one another, they are not malleable. This causes ionic solids to be hard and brittle.
  3. Because the ions are attracted to the δ+ and δ− ends of water, they have high solubility in water.
  4. Since ionic bonds are strong, their crystals are hard and the compounds have high melting and boiling points.


Molecular (covalent) solids such as I[sub]2[/sub] and SiO[sub]2[/sub] gave relatively weak intermolecular forces between molecules.

In general, their physical properties are as follows:
  1. Because there are no free moving electrons or ions, they do not conduct heat and electricity, ever.
  2. Because particles are "glued" in position and cannot slide over one another, they are not malleable.
  3. In general, covalent compounds have low solubility in water.
  4. The intermolecular forces between covalent compounds are relatively weak and easy to break. Therefore, solid covalent compounds are soft, and the compounds have low melting and boiling points.


Metallic bonds are formed when valence electrons are mobile. Example: Al (s)

Metals such as Cu(s) and Au(s) have metallic bonds between the particles. Valence electrons in the metals are not confined to each atom but are shared by all. Therefore, the positive ions (cations) are said to be immersed in a "moving sea" of electrons.

The physical properties of metals are as follows:
  1. Because there are free moving electrons present, they are good conductors.
  2. Because the positive ions can slide over one another, metals are malleable and ductile.
  3. They have low solubility in water.
  4. Since metallic bonds are also strong, they are hard and the atoms have high melting and boiling points.



NaCl:
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Metals are malleable because the free moving electrons act as a lubricant and allow the positive ions to slide over each other.

However, in ionic solids, the cations and anions are held in rigid positions. When a force is applied to the crystal, like charges line up. When similar ions are close to each other, they repel one another, causing the crystal to shatter.