Which of the following materials is likely to have the lowest thermal conductivity? a) Iron, b) Copper, c) Gold, d) Wood, e) Tin.
Looking over this list of possible choices, we notice that four of these five options are metals. That is iron, copper, gold, and tin. These are all metal materials. When it comes to their conductivity properties, both in terms of conducting electricity or for thermal conductivity, what makes metals stand out is that they have excess loosely held electrons.
For example, let’s consider a copper atom. If we look up copper on the periodic table of elements, we see that it has an atomic number of 29. That means that a neutral copper atom has 29 electrons. There’s two in its innermost shell and then eight in the next shell out. Then, in the shell outside, that there are 18 electrons. And if we count all those electrons up that we’ve added it so far, they add up to 28, which means that we have just one single solitary electron out here in this outermost shell.
This electron, because there’s just one of them in this energy level and because it’s so far from the nucleus, is prone to be shared with other atoms. It’s easily pulled away from this particular one. So, when we put a whole bunch of copper atoms together to form a bulk material, like a wire, each atom contributes a single electron which is free to move about this bulk structure.
These electrons are wonderful for conducting electricity. And they’re also effective at conducting heat, thermal energy. Because these electrons are so mobile, they’re able to pick up energy from one part of the structure and move it to another part. By doing this, they significantly increase the conductivity, both thermal and electrical, of a material.
In addition to copper, the other metals on this list, iron, gold, and tin, operate in a similar way. Their molecular structure involves loosely held electrons which are able to aid in the conductivity of thermal energy. In other words, these metals have very high thermal conductivity. But in this question, we’re asked which material probably has the lowest thermal conductivity.
From that perspective then, we’ll look for the one material which is not a metal. That is the one which doesn’t have the advantage of many freely moving electrons to help its conductivity along. And we can see that that material is choice d wood. The conductivity through wood is not helped along by loosely held electrons, as it is in metals. Therefore, we expect it to have the lowest thermal conductivity of this set of materials.