True or False? Please give a short 1 to 2 sentences on why you think it would be

Question

True or False? Please give a short 1 to 2 sentences on why you think it would be true or false. Most metals and alloys form close-packed structures such as face-centered cubic and hexagonal close-packed, because metallic bonding is isotropic, the atoms behave as hard spheres, and these crystal structures have the highest atomic packing density. Covalently bonded elements and alloys tend to form tetrahedrally coordinated structures such as diamond cubic and zinc-blende. because they share four electrons on average in sp3-type hybridized bonding. Key factors in determining what crystal structures ceramics will form are the magnitude of the electrical charge on each of the component ions and the relative sizes of the cations and anions. Some materials that have anisotropic crystal structures may display relative isotropic properties in polycrystalline form. The difference between crystalline and non-crystalline materials is the absence of long-range atomic order in the former. X-rays and electrons are used to determine the structure of materials because they have wavelengths that are shorter than the interatomic spacings in materials.

Explanation / Answer

1)true they are closelym packed and have high atmoic density 2)false Within diamond, one s-orbital and three p-orbitals undergo a SP3 hybridization. 3)true Stable ceramic crystal structures: anions surrounding a cation are all in contact with that cation. For a specific coordination number there is a critical or minimum cation anion radius ratio rC/rA for which this contact can be maintained. 4)false Fiber reinforced composite materials typically exhibit anisotropy. That is, some properties vary depending upon which geometric axis or plane they are measured along. For a composite to be isotropic in a specific property, such as CTE or Young’s modulus, all reinforcing elements, whether fibers or particles, have to be randomly oriented. This is not easily achieved for discontinuous fibers, since most processing methods tend to impart a certain orientation to the fibers. 5)true A solid is crystalline if it has long-range order 6)true ..fact

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