Structures of solids

Solids have properties that are extremely different from those of gases, e.g., solids are incompressible and have definite shapes.  These properties result from the arrangement of constituent particles (molecules, atoms or ions).  In solids, the particles are in regular patterns and are so close together that they exert very powerful forces on one another.

Types of solids

1.  Molecular solids.

2.  Covalent (atomic) solids.

3.  Ionic solids.

4.  Metallic solids.

1.  Molecular solids:

These consist of discrete (simple) molecules held together by weak forces, e.g., Iodine, paraffin wax and gases such as noble gases, halogens, oxygen, carbon dioxide, nitrogen, etc.  When they solidify, molecular solids are volatile and have low melting points and heats of vaporization since their molecules are held together by weak forces.  They do not conduct electricity.

2.  Covalent (atomic) solids: 

These consist of atoms joined by single covalent bonds, e.g., diamond, silicon, silicon dioxide (silica), and silicon carbide.

Structure of diamond

Diamond, an allotrope of carbon contains a three – dimensional array of carbon atoms with valence bonds directed toward the apices of a regular tetrahedron (fig. 5.1).  The whole structure is one giant molecule and, because the bonding is strong and extended in three dimensions, diamond is exceptionally hard.  The melting point is 3500⁰ C.

 Structure of silicon dioxide

Silicon dioxide is also a three-dimensional giant molecule and one form of this compound (cristobalite) has a structure in some ways similar to that of diamond.  Each silicon atom is surrounded tetrahedrally by four oxygen atoms, i.e., each oxygen atom is shared equally between two silicon atoms giving the empirical formula, SiO₂, and melting point 1700⁰C. Atomic solids do not conduct electricity.

Trial 1

 Explain the following observations:

(c) Carbon dioxide is a gas at room temperature whereas silicon (IV) oxide is a solid of high melting point.                                                                                                                                               (04marks)

(d) Diamond is a hard substance but graphite is soft and slippery.

Ionic solids

These consist of infinite arrays of positive and negative ions held together by electrostatic forces (giant structures), e.g., sodium chloride. Ionic solids conduct electricity in molten and solution forms.

The sodium chloride structure

The x-ray analysis of crystal structures shows that sodium chloride consists of a regular three-dimensional assembly of Na⁺ and Cl^– ions.

Each sodium ion is surrounded by six Cl^– ions as its neighbors and each Cl^– is surrounded by six Na⁺ ions.  Therefore, both the sodium ion and chloride ions in the structure have a coordination number of six. Sodium chloride is an ionic compound that conducts electricity in molten and solution forms.

Trial 2

Fig.5.4 below shows the lattice structure of an ionic salt, MX.

 (a) State the coordination number of M⁺ and X^– ions. Explain your answer.             (3 marks)

(b) Name the type of lattice structure shown in the diagram above.                         (1 marks)

Metallic solids

These consist of infinite arrays of bonded atoms except that the atoms are not bonded together by covalent but by metallic bonds.

Fig. 5. Cross-section of a metallic crystal.

Each encircled positive charge represents the nucleus and the field of non valence electron shells of metal atoms.  The dash (-) represents released mobile valence electrons into the electron pool.  It is the attractions between the positive ions and electron pool that keeps (bonds) the atoms together in a metallic crystal.

Two-dimensional structures

The layer lattice structure of carbon (graphite)

Graphite contains layers of carbon atoms; each carbon atom in a particular layer is covalently bonded to three others, giving C-C-C bond angle of 120⁰.  The individual layers are held together by van der Waals’ forces.

Physical properties arising from a layer lattice structure

1.  Graphite is soft and greasy:  This is because the layers can slip over each other easily since they are held by weak forces.    

2.  The density of graphite is lower than that of diamond: This is because the graphite structure is much more open than that of diamond.

3.   Graphite is a conductor of electricity but the diamond is not:  This is because each carbon atom in the graphite structure has one unused p-electron (π- electrons). These electrons can move freely between the layers; it is due to these mobile electrons that graphite is an electrical and thermal conductor.

Summary

The summary of the properties of different solids is given in table 1.

Types of solid	Constituents	Binding forces	Properties	Example
Molecular solids	Simple molecules	van der Waals’ Dipole-dipole Hydrogen bonds	Volatile Insulators	Noble gas CH4, CO2, Cl2
Ionic solids	ions	Electrostatic attractions between cations and anions	Non-volatile Conduct electricity  in molten and solution forms conduct electricity Have high mpt and bpt	NaCl, KCl
Covalent solids	atoms	Covalent bonds	Very high bpt and mpt. Insulators	Diamond Silicon, SiO2
Metallic solids	atoms	Metallic bonds or molecular orbitals overlap in crystal	Conductors in molten and solid forms Moderately high mpt/bpt.	Na, Cu, Fe

Trial 3

Complete the table below about the properties of different types of crystals (full page) (9marks)

Types of crystals	Forces holding the crystals	Melting points (state whether low, moderate, high or very high	Form in which electricity is conducted if any
Metal
Ionic
Network covalent

Suggested answers to the trials

Trial 1

• Carbon dioxide has simple molecules that are bonded by weak van der Waals forces which can easily be overcome at a temperature below room temperature and is a gas at room temperature. Silicon dioxide is a giant molecule in which each silicon atom is bonded to four oxygen atoms and each oxygen atom is bonded to two silicon atoms by strong covalent bonds that require high temperature to be broken. The reason why it is solid.

• Graphite consists of hexagonal layers of carbon where each carbon atom is covalently bonded to 3 others. The layers are held by weak forces that allow the layers to slide over one another making it slippery. Diamond has a giant structure in which strong covalent bonds extend in a 3-dimensional network giving the diamond a very hard structure.

Trial 2

• The coordination number of each ion is six because each ion is surrounded by six opposite ions.
  • Ionic lattice.

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Compiled by Dr. Bbosa Science