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.

2.1 Metallic bonding