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Transcript
Topic 4: Materials Metals are often described as positively charged nuclei in a sea of electrons. The outer electrons of the metal atom nuclei are free and can flow through the crystalline structure. The bonding is caused by attraction between the positively charged metallic atom nuclei and the negatively charged cloud of free electrons. Specific arrangements of metal atoms are not required. Metallic bond Metallic bonding involves sharing outer electrons but these are freer and they can flow through the crystalline structure. The bonding is caused by attraction between the positively charged metal atom nuclei and the negatively charged cloud of free electrons, and is spread throughout the lattice – “positively charged nuclei in a sea of electrons”. How the movement of free electrons makes metals very good electrical and thermal conductors Movement of free electrons In a metal, the outer electrons are shared among all the atoms in the solid. Each atom gives up its outer electrons and becomes slightly positively charged. The negatively charged electrons hold the metal atoms together. Since the electrons are free to move, they lead to good thermal and electrical conductivity. With the metallic bond, atoms are also held together by their outermost electrons, but instead of being shared by a couple of atoms, these electrons act like a cloud that is able to move through the structure of the crystal. This cloud, which is negatively charged (electrons have a negative charge), pulls on the atoms, which are positively charged (each atom is missing one or more of its electrons). The attraction between the atoms extends beyond the crystal, pulling in on adjacent crystals. The electron cloud moves freely when + ‐ a voltage is applied. This movement of electrons is an electric current. The electron cloud can also transfer thermal energy, making it a good conductor of heat. Also, the electron cloud absorbs then radiates most of the light that falls on it, which gives metal its lustrous appearance. Crystal structure Pure and alloyed metals exist as crystals Metals, many ceramics and some polymers have a uniform, geometrical arrangement of atoms or ions that is repeated throughout the material and are therefore said to be crystalline. This regular 3‐D atomic pattern is known as the space lattice and the unit cell is the smallest unit of the lattice that retains the overall characteristic of the lattice. The crystal structure refers to the size, shape and atomic arrangement of the lattice unit cell and varies from one substance to another. Crystals are regular arrangements of particles (atoms, ions or molecules). 82