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Solids - Crystallization and Bonding Help

By — McGraw-Hill Professional
Updated on Aug 28, 2011

Crystallization And Bonding

There are four different types of bonding that occur in crystalline solids. These determine what type of solid it is.

The four types of solids are molecular, metallic, ionic , and covalent .

Molecular Solids

These types of crystalline solids have molecules at the corners of the lattice instead of individual ions. They are softer, less reactive, have weaker non-polar ion attractions, and lower melting points.

A molecular solid is held together by intermolecular forces. The bonding of hydrogen and oxygen in frozen water shows how hydrogen forms bonds between different water molecules.

Figure 15.2 shows the cubic arrangement of sodium chloride.

Solids Molecular Solids

Fig. 15.2. The cubic arrangement of sodium chloride gives an example of intermolecular forces.

Metallic Solids

Another type of crystalline solid is made up of metals. All metals, except mercury, are solid at room temperature. The temperature needed to break the bonds between positive metal ions in specific lattice positions, like iron in iron(II) disulfide (FeS 2 ), and the happy valence electrons around them is fairly high. This strong bonding gives their stable molecules flexibility and allows them to be formed into sheets and strands without breaking as we learned in Chapter 12.

A metallic solid like iron or silver is held together by the “bread pudding” type of bonding. When a positive central core of atoms is held together by a surrounding general pool of negatively charged electrons it is called metallic bonding . This arrangement of (+) metal ions and delocalized valence electrons makes them good conductors of electricity.

Ionic Solids

Ionic solids form a lattice with the outside points made up of ions instead of larger molecules. These are the “opposites attract” solids. The contrasting forces give these hard, ionic solids (like magnetite and malachite) high melting points and cause them to be brittle. Figure 15.3 shows the melting points of different solids.

Solids Molecular Solids Ionic Solids

Fig. 15.3. Depending on bonding strength, solids have a wide variety of melting points.

Ionic bonding in a solid occurs when anions (–) and cations (+) are held together by the electrical pull of opposite charges. This electrical magnetism is found in a lot of salts like potassium choride (KCl), calcium chloride (CaCl), and zinc sulfide (ZnS).

Ionic crystals that contain ions of two or more elements form three-dimensional crystal structures held together by strong ionic bonds.

Additionally, ionic compounds are electrolytes . Electrolytes form ions and carry a current when melted or dissolved in a solvent during electrolysis .

Electrolysis is the method of breaking down an electrolyte by separating its ions between positive and negatively charged electrodes.

Positive cations move to the cathode (+) and gain electrons, while negative anions move toward the anode (–) and lose electrons.

Covalent Solids

A grouping of covalent bonds holds some solids together. Assembled together in large nets or chains, covalent multi-layered solids are extremely hard and stable in this type of configuration. For example, diamond atoms use this type of structure when they arrange into three-dimensional solids. One carbon atom is covalently bonded to four other carbons. This strong crystalline structure makes diamond the hardest known organic solid.

Covalent crystals are all held together by single covalent bonds. This type of stable bonding produces high melting and boiling points.

The different bonding and forms of carbon in a diamond (pyramid shaped), graphite (flat layered sheets), or buckminsterfullerene (C 60 and C 70 , shaped like a soccer ball) give an idea of the variety and stability of covalent molecules. Nets, chains, and “balls” of carbon bonded into stable molecules make these solids hard and stable.

Allotropes are different structural forms of the same element. Graphite, diamond, and buckminsterfullerene are all allotropes of carbon.

Practice problems for these concepts can be found at –  Solids Practice Test

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