When we discuss solids, therefore, we consider the positions of the atoms, molecules, or ions, which are essentially fixed in space, rather than their motions which are more important in liquids and gases. The faces of crystals can intersect at right angles, as in galena PbS and pyrite FeS 2 , or at other angles, as in quartz. Right Cleavage surfaces of an amorphous solid.
Obsidian, a volcanic glass with the same chemical composition as granite typically KAlSi 3 O 8 , tends to have curved, irregular surfaces when cleaved. Crystalline solids, or crystals, have distinctive internal structures that in turn lead to distinctive flat surfaces, or faces.
The faces intersect at angles that are characteristic of the substance. When exposed to x-rays, each structure also produces a distinctive pattern that can be used to identify the material. The characteristic angles do not depend on the size of the crystal; they reflect the regular repeating arrangement of the component atoms, molecules, or ions in space. When an ionic crystal is cleaved Figure In a covalent solid such as a cut diamond, the angles at which the faces meet are also not arbitrary but are determined by the arrangement of the carbon atoms in the crystal.
Figure Deformation of the ionic crystal causes one plane of atoms to slide along another. The resulting repulsive interactions between ions with like charges cause the layers to separate. Crystals tend to have relatively sharp, well-defined melting points because all the component atoms, molecules, or ions are the same distance from the same number and type of neighbors; that is, the regularity of the crystalline lattice creates local environments that are the same.
The most discussed example of a hydrogen bonding crystalline solid is ice. In ice, the oxygen atom is surrounded by four hydrogen atoms at the corner of the tetrahedron. A list of organic materials like alcohol , carboxylic acid, proteins is also an example of hydrogen bonding crystalline solids.
Forces involved in ionic crystalline solid are electrostatic in nature. These are strong and non-directional types. Therefore, ionic crystalline solid is strong and likely to be brittle. They have very little elasticity and cannot be bent easily. The melting point in ionic crystals is high which is decreases with increasing the size of the ions.
Sodium chloride NaCl , potassium chloride KCl , magnesium chloride MgCl 2 are the most common examples of ionic crystals. Calcium carbonate CaCO 3 is an example of an ionic crystal where some atoms are held together by covalent bonding.
In many crystals, the atoms in the structural units are held together by covalent bonding by pairing electrons of hybridized orbitals to form giant type molecules.
Diamond, germanium, zinc sulfide, silver iodide, silicon carbide are the known examples of covalent crystals. In a diamond, every carbon atom is covalently linking with the other four carbon atoms along the tetrahedron.
Electrons are held loosely bound in these types of crystalline solid. They are good conductors of electric energy and thermal energy. Metallic crystalline solid is strong but can be bent. The metal atom mainly formed cubic body-centered, face-centered, and hexagonal closed packed crystalline solid.
In hexagonal crystalline form, every metal atom is surrounded by 12 other metal atoms by the metallic bond with coordination number twelve. The forces are non-directional in nature. All the metals of periodic table elements are formed different types of crystals structure. In crystalline solids, each of these unit cells contains the same atoms or molecules arranged in the same way. The contents may be oriented in various ways in the special case that symmetry elements are present.
It is these ordered fillings of lattices that create the crystalline state. The lattice structure, unit cell and unit cell contents and composition are determined through a diffraction experiment, usually using X-rays, but neutron diffraction is used in certain cases.
The wavelength of the X-ray chosen is small enough to determine the distances between atoms in a unit cell. Using diffraction, scientists can determine the chemical structure of molecules, metallic materials, ceramic materials and even biological molecules such as DNA or proteins.
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Covalent network crystals — A covalent network crystal consists of atoms at the lattice points of the crystal, with each atom being covalently bonded to its nearest neighbor atoms see Figure below. The covalently bonded network is three-dimensional and contains a very large number of atoms. Network solids include diamond, quartz, many metalloids, and oxides of transition metals and metalloids. Network solids are hard and brittle, with extremely high melting and boiling points.
Being composed of atoms rather than ions, they do not conduct electricity in any state. Diamond is a network solid and consists of carbon atoms covalently bonded to one another in a repeating three-dimensional pattern. Each carbon atom makes four single covalent bonds in a tetrahedral geometry.
Molecular crystals — Molecular crystals typically consist of molecules at the lattice points of the crystal, held together by relatively weak intermolecular forces see Figure below.
The intermolecular forces may be dispersion forces in the case of nonpolar crystals, or dipole-dipole forces in the case of polar crystals. Some molecular crystals, such as ice, have molecules held together by hydrogen bonds.
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