Video Transcript
Which of the following giant structures does not conduct electricity? (A) Molten magnesium oxide, (B) graphite, (C) diamond, or (D) sodium chloride dissolved in water.
The question asks, which of the following does not conduct electricity? Let’s first discuss what kind of substances do conduct electricity. And in this way, we can rule out some answers.
Electricity could simply be defined as the flow of charge or electric charge. For charge to flow, this requires movable charges, such as electrons or ions, and a complete circuit. Lastly, sometimes a source of energy and a potential difference is needed to induce or cause the flow of charges in the circuit. In a moment, we will use this simple diagram with a battery as a source of energy to see if electricity flows through each of these substances.
The question also mentions that all these substances are giant structures. Let’s look at each. Magnesium oxide in the solid state is a giant ionic structure. Its ions are fixed in place and held by strong electrostatic forces. However, in the molten state, its ions are free to move. The diagram shows electricity flowing through molten magnesium oxide. Because the ions are free to move, the positively charged magnesium ions and the negatively charged oxide ions flow in different directions to complete the electrical circuit. So molten magnesium oxide is not a possible answer because it does conduct electricity due to the free-moving ions.
Sodium chloride in its solid state is also a giant ionic structure. In this solid, the ions are held strongly or tightly in place. But when dissolved in water or aqueous, its ions are free to move. The diagram shows a solution of aqueous sodium chloride. And the positively charged sodium ions and negatively charged chloride ions are free to move and move in different directions to complete the electrical circuit. Because sodium chloride dissolved in water does conduct electricity, it is not a possible answer.
Now, let’s look at graphite and diamond, which are both giant covalent structures. In each, carbon is covalently bonded to other carbons. Carbon atoms are not free to move when they are bonded covalently. So let’s see if electrons are free to move. If we could look inside a graphite rod, we would see a honeycomb structure, where each corner represents a carbon atom. And each carbon atom drawn with the pink dot is bonded to three other carbon atoms. The bonds between each carbon atom are covalent.
Carbon has four valence electrons, which means it can potentially bond to four different atoms. But in graphite, each carbon atom is only bonded to three other atoms. So one of the four valence electrons remains unbonded. These honeycomb structures exist in flat sheets. So if we were to look at the side view of graphite, we would see what looks like layers on top of each other.
The single unbonded electron from each carbon atom is free to move between the sheets or layers of graphite. We say these electrons are delocalized. So if we inserted a graphite rod into a circuit with a battery, the circuit would indeed conduct electricity because of these free-flowing and delocalized electrons between the sheets of graphite. We can therefore say that graphite is not a possible answer because the question asked, which structure does not conduct electricity?
Finally, let’s have a look at diamond. We know that carbon can make four bonds. And in diamond, each carbon atom is bonded to four other carbon atoms. On and on, the network goes in three dimensions to produce a giant covalent structure. It is difficult to draw each carbon with four bonds. So this diagram only shows the beginnings of a network. Nevertheless, each carbon atom is strongly covalently bound to neighboring carbon atoms and are not free to move, nor are there any delocalized electrons free to move and conduct electricity. So in the circuit diagram drawn, electrical current will not flow through the circuit because there are no movable charges.
Finally, the giant structure which does not conduct electricity is diamond.
