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## The nature of the metallic bond

The structure of a metallic bond is quite different from covalent and ionic bonds. In a metallic bond, the valence electrons are delocalised, meaning that an atom's electrons do not stay around that one nucleus. In a metallic bond, the positive atomic nuclei (sometimes called the “atomic kernels”) are surrounded by a sea of delocalised electrons which are attracted to the nuclei (see figure below).

A microscopic model of metals

Definition 1: Metallic bond

Metallic bonding is the electrostatic attraction between the positively charged atomic nuclei of metal atoms and the delocalised electrons in the metal.

## Properties of metals

1. Metals are shiny.

2. Metals conduct electricity because electrons are free to move.

3. Metals conduct heat because the positive nuclei are packed closely together and can easily transfer the heat.

4. Metals have a high melting point because the bonds are strong and a high density because of the tight packing of the nuclei.

### Activity 1: Building models

Using coloured balls (or jellytots) and sticks (or toothpicks) build models of each type of bonding. Think about how to represent each kind of bonding. For example, covalent bonding could be represented by simply connecting the balls with sticks to represent the molecules, while for ionic bonding you may wish to construct part of the crystal lattice.

Do some research on types of crystal lattices (although the section on ionic bonding only showed the crystal lattice for sodium chloride, many other types of lattices exist) and try to build some of these. Share your findings with your class and compare notes to see what types of crystal lattices they found. How would you show metallic bonding?

### Exercise 1: Bonding

Give two examples of everyday objects that contain:

1. covalent bonds

2. ionic bonds

3. metallic bonds

Learners can give any reasonable suggestion, some ideas are given below:

a) Graphite in pencils, water, ammonia in cleaning fluid

b) Table salt, chalk, some rocks

c) Metal cutlery, metal jewellry

Complete the table which compares the different types of bonding:

Table 1
 Covalent Ionic Metallic Types of atoms involved Nature of bond between atoms Melting point (high/low) Conducts electricity? (yes/no) Other properties
 Covalent Ionic Metallic Types of atoms involved Mainly non-metals Some non-metals and some metals Metals only Nature of bonds between atoms Sharing of electrons Transfer of electrons Delocalisation of electrons Melting point (high/low) Low High High Conducts electricity? (yes/no) No Only when molten or in solution Yes Other properties Poor conductor  of heat Brittle Malleable, shiny, ductile

Complete the table below by identifying the type of bond (covalent, ionic or metallic) in each of the compounds:

Table 2
 Molecular formula Type of bond $H 2 SO 4$ $FeS$ $NaI$ $MgCl 2$ $Zn$

Zinc will conduct electricity most effectively as it is metallically bonded which means that it has a 'sea' of electrons around it. These electrons can move and this allows conduction of charge.

Use your knowledge of the different types of bonding to explain the following statements:

1. A sodium chloride crystal does not conduct electricity.

2. Most jewellery items are made from metals.

3. It is very hard to break a diamond.

4. Pots are made from metals, but their handles are made from plastic.

a) In a solid ionic substance there are no free electrons and so charge cannot flow.

b) Metals are shiny and malleable. The malleability means that it can be formed into many shapes. Jewelry is designed to be pretty and attractive and so shiny materials would be preferred over non-shiny materials.

c) Diamond is covalently bonded. Covalent bonds are very strong bonds and it is hard to break them.

d) Metal is a good conductor of heat, while plastic is a poor conductor of heat. When you cook food you need the pot to be a good conductor of heat but the handles should be poor conductors of heat so that you don't burn yourself.