|MadSci Network: Chemistry|
Good question. You are right - if there were not some forces holding the particles together, gravity would pull them apart and all solids would collapse. So there obviously must be some forces holding the particles together. These forces can be chemical, electrical, or mechanical. Mechanical forces can hold particles together by friction or interlocking edges, for example. A large scale example of friction would be how rough bricks can be stacked into a jumbled heap without the whole pile collapsing. Interlocking particles are similar to the way jigsaw puzzle pieces connect and hold each other in place. These forces can apply on microscopic to large scale depending on the size and shape of the particles. Chemical forces also hold solids together. Depending on the material, there are variety of different kinds of chemical bonds that can form between the individual particles. It all depends on how the electrons in the individual molecules interact. Electrical forces can hold particles together by the attraction between positively-charged particles and negatively charged ones. Which forces are most important in a particular solid depends on the materials (materials) involved, the size and shape of the individual particles, and the conditions (like temperature and pressure). It is the combination of the various forces and conditions that given solids their differnt properties.
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