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Learn about the tetragonal crystal system, one of the 7 crystal systems in crystallography. Find out its Bravais lattices, point groups, space groups, mineral examples and related topics.
A fixation disparity is not constant within a certain observer, but can vary depending on the viewing conditions. If test prisms with increasing amount are placed in front of the observer’s eyes, the fixation disparity changes in the eso direction with base-in prisms and in the exo direction with base-out prisms (Fig. 3).
A base is a substance that can accept hydrogen ions (protons) or donate electron pairs to other substances. Learn about the different types of bases, their reactions with water and acids, and their applications in chemistry.
Learn about the shape and symmetry of chemical compounds with the formula ML5, where L is a ligand. Find examples of square pyramidal molecules, such as chlorine pentafluoride and xenon oxytetrafluoride, and their relation to trigonal bipyramidal geometry.
A trigonal pyramid is a molecular geometry with one atom at the apex and three atoms at the corners of a trigonal base. The bond angle of a trigonal pyramid is 90°, but it is distorted to 107° by the lone pair of electrons on the central atom.
The Maddox rod test is a subjective method to measure horizontal and vertical deviations of the eyes, using a red line and a white light. The double Maddox rod test can also assess torsion and cyclotropias, by comparing the angles of two red lines.
Learn about the shape and symmetry of compounds with eight atoms or ligands around a central atom, such as XeF2− 8 and ReF− 8. See examples, distortions, and comparisons with other geometries of eight-coordination.
A formula unit is the smallest unit of a non-molecular substance, such as an ionic compound, covalent network solid, or metal. It can also refer to the chemical formula for that unit. Learn more about formula units, examples, and how they are used in stoichiometry and mineralogy.