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What is a metal?
What is a metal?

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2.1 Metallic bonding

The attraction between the delocalised electrons and the positively charged nuclei is called metallic bonding. The metallic bonding is strong and occurs in all directions. Breaking this attraction is difficult, and this is the reason that metals have high melting temperatures.

Metals are good conductors of electricity and heat, because the free moving electrons facilitate the transfer of charge or heat through the material.

Figure 5 shows a simple electric circuit with a metal wire, a battery and a bulb. When the wire is connected between the positive and negative terminals of the battery, one end of the wire becomes positively charged and the other becomes negatively charged. This causes the electrons, which are free to move, to travel through the wire towards the positive terminal of the battery, where they are removed. At the same time the negative terminal supplies more electrons to the wire.

Within the battery there is a net flow of negative charge from the positive terminal to the negative terminal, which balances the flow through the wire, so that charges don’t continually build up at the battery terminals.

You may be familiar with the term ‘voltage’. The voltage of the battery can be considered as the ‘push’ exerted on electrons moving along the circuit and the flow of negatively charged electrons in the wire constitutes the electric current. Note that in Figure 5 the arrows refer to the flow of electrons. By convention, the direction of the electric current is in the opposite direction, from the positive terminal towards the negative terminal of the battery.

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Figure 5  Conductivity of electricity in a metal: (a) open circuit (switch open) (b) closed circuit (switch closed). Arrows indicate flow of electrons.
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Two simple, rectangular electric circuit diagrams show the conductivity of electricity in a metal. Figure 5a shows an open circuit. Straight lines are used to represent a connecting metal wire. The circuit has a gap at the top to indicate a break in the circuit, with a switch labelled ‘open’. To the right of this open switch, there is a rectangular block that represents a battery with its positive (+) terminal next to the switch and its negative (–) terminal on the far right. On the right side of the circuit, there is a circle with a squiggly line in it that represents a light bulb and its filament. Figure 5b shows a closed circuit. The same circuit as in (a) has the switch closed at the top so that the wire has contact with and is connected to the positive (+) terminal of the battery. There are circles, indicating freely moving electrons, around the circuit. There is a lit bulb on the right. Arrows indicate the clockwise flow of the negatively charged electrons in the wire from the negative terminal to the positive terminal of the battery. What is happening within the wire is shown with a magnified diagram. This is a repeat of Figure 1.4. Each nucleus is represented by a circle with a plus symbol (‘+’) in it. The nuclei are arranged in staggered rows. That is, each row of circles in alternate rows are offset by half a circle, to the left or the right, so that they fit in between the circles in the adjacent rows. The electrons in the outer shells are represented by blue circles that are positioned randomly, indicating they are moving freely because they are delocalised.

Figure 5  Conductivity of electricity in a metal: (a) open circuit (switch open) (b) closed circuit (switch closed). Arrows indicate flow...

  • When will the bulb light?

  • The bulb will light only when the circuit is closed and there are electrons flowing through it.

The battery voltage depends on the choice of chemicals inside it.

The delocalised electrons also explain other metallic characteristics such as malleability. The bonding occurs in every direction throughout the metal enabling atoms to roll easily over each other without breaking any bonds when stress is applied (Figure 6).

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Figure 6 Layers of atoms sliding over each other and creating thin layers of metal.
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An animation showing how thin layers of metal are created. The animation begins with four stacked rows of orange spheres, which represent atoms in the metal. Each sphere in alternate rows are offset by half a sphere, to the left of the right, so that it fits into the space between adjacent spheres in row above. The animation shows the top two rows of atoms sliding across the bottom two, towards the right. An arrow indicates stress being to the top two rows of atoms to cause this sliding.

Figure 6 Layers of atoms sliding over each other and creating thin layers of metal.