What makes materials different from each other?

Nov 06 2013

On a basic level, science is about asking why the world is the way it is, and engineering is about asking, how can we use that to better our condition? There is certainly a lot of interplay between the two; they inform each other and rely on each other. And in my mind, a good scientist should always be a reasonable engineer, and vice versa. So if we want to understand what the science is that underpins a lot of our current technology, we first have to ask a lot of “why” questions about the world around us. Such as, why do different objects and materials have different properties? Why are there different forms that matter can take? Why do some forms appear on Earth and some don’t? Then, once we know what makes materials different from each other, we can start talking about how to use that.

So what is the difference between the atoms in a metal table, the atoms in a cup of coffee, and the atoms in our hands? There are two major differences that are relevant: first, the atoms themselves come in a wide variety of types, and second, they can be arranged with other atoms in many unique ways that affect the property of the resultant material. The image below shows how we can arrange the same silicon and oxygen atoms in a random way or an ordered way, to get either silica or quartz. This change in order affects the physical properties of the resultant material.

 

One way atoms can differ is by having different numbers of protons, neutrons, and electrons. Proton number affects electron number, because of the attractive force between protons and electrons due to their opposite charge. So, for a given number of protons, an atom will end up with a similar number of electrons. How many electrons an atom has is very important, because the cloud of electrons is much larger than the compact nucleus which contains the neutrons and protons, so electrons are the primary means by which an atom interacts with the world.

What “the world” means here is primarily other atoms. So to assemble a solid, we have lots of atoms whose electron clouds are interacting with each other. Atoms can share electrons, they can be attracted to each other if they have opposite charges, and they can form three-dimensional structures to allow many atoms to interact . These interactions are all based around electronic forces, which stem from charge as we discussed earlier. Different kinds of atoms will experience different forces in different environments, so we end up with a whole slew of ways to assemble atoms. We can pack carbon into sheets and get pencil lead, we can jam it together with no ordering and get charcoal, we can compress it until it has a dense, flawless periodic structure and get diamond, we can mix it with hydrogen to get the long hydrocarbon chains that crude oil is made of, or we can isolate sheets of it to get the wonder nanomaterial graphene. And that’s just carbon!

The different ways that materials can be put together from atoms is really important. For example, it affects how easily materials conduct heat and how much they expand or contract as temperature changes. The whole diversity of materials that you see around you, from the clear glass in a window to a solid wooden table to a metal pen and the ink inside, are all different because of their differing atomic structure! And understanding where those differences come from is a major part of physics.

This is based on a piece which originally appeared at Let's Talk About Science.

Author

Research Fellow, School of Chemistry & CRANN, Trinity College Dublin

Dart of Physics Blog

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