#7. The Way a Material Behaves Depends on how its Atoms are Arranged.

Professor James Trefil (author of Science Matters, Why Science?, and 30 other books on science literacy) identified 18 key science concepts every adult should know to be a science literate. We’re here to reintroduce adults to science, in a fun way! It’s all part of our Brain Makeover project to increase adult science literacy. Here’s concept #4, presented by 76ers Cheerleader Lauren and explained by Professor James Trefil.  We’ll post one each week (more or less) and it to the Brain Makeover collection.
The properties of a material depend on the type of bond holding atoms together as well as the arrangement of those atoms. For example, ionic bonds are often found in materials like minerals and ceramics, covalent bonds in the molecules of living systems, and metallic bonds (as the name implies) in metals. The difference between diamond and graphite—both made completely of carbon atoms–depends on the fact that in a diamond those atoms are arranged in a way that all the bonds of covalent, while in graphite some of the bonds are weaker, depending on polarization forces.
The electrical properties of materials depend on how strongly electrons are locked into their bonds. In a metal, for example, electrons are free to respond to outside forces. Such a material is called a conductor, because it allows electrical current to flow. In a plastic or ceramic, on the other hand, electrons are locked tightly into covalent or ionic binds and are not free to move. Such materials are called insulators.
In the twentieth century two other kinds of materials were discovered. Superconductors are materials through which electrical current can flow forever without loss. Normally, materials are superconducting only at low temperatures. Superconducting magnets are used extensively in MRI machines.
Semiconductors (like silicon) are materials in which electrons  are occasionally shaken loose from covalent bonds, and so are available to carry electrical current. Two layers of semiconductors are the basic for solar photovoltaeic cells, in which sunlight shakes loose electrons to create electrical current. A transistor is a triple layer of semiconducting materials and is the basis of modern computers.

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