4.2 MATERIALS 76 See unit 2.1 for more information. There exist 118 known elements, which are arranged in the periodic table according to their atomic number, i.e. the charge number of its atomic nucleus. CLASSIFICATION AND PROPERTIES Matter, made up of atoms which compose elements , is the content of all the materials around us. These can be divided into five broad categories: metals, polymers, ceramics, composites, and smart materials. Their properties determine characteristics that make them suitable or not for certain applications. Mechanical properties Mechanical properties define the ability of a material to resist an external force (for example a load) or a stress. The main mechanical properties are the following. Thermal properties Thermal properties refer to the response of materials to thermal changes. The main thermal properties are: thermal conductivity: the ability to conduct heat; thermal expansion: the tendency of a material to increase its size and density in response to an increase in temperature; melting point: the temperature at which a material melts and changes its status from solid to liquid. Gallium Electrical properties Electrical properties refer to the response of materials to electricity. to melt: fondere to regain: riacquistare scratch: graffio to stretch: allungare to undergo: subire to withstand: resistere 184 MeCHaNiCs strength is the ability to withstand forces without breaking. hardness is the ability to resist abrasion and scratches. stiffness is the ability to resist deformation. elasticity is the ability to regain the original shape when the force is removed. ductility is the ability to be deformed when stretched. plasticity is the ability to deform permanently without breaking. malleability is the ability to be deformed under compression and take on a new shape. brittleness is the property of breaking when subjected to stress. Conductivity is the ability to conduct electricity. Resistivity is the ability to resist the flow of electricity through itself. Chemical properties Chemical properties refer to the way a material reacts when it undergoes a chemical reaction or change. The main chemical properties are: corrosion resistance, the resistance to oxidation; density, the relationship between the mass and the volume of the material. Atomic volume is an important parameter to determine it.