Examples of unit cell in the following topics:
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- The most common way to describe a crystal structure is to refer to the size and shape of the material's characteristic unit cell, which is the simplest repeating unit within the crystal.
- In principle, one can reconstruct the structure of an entire crystal by repeating the unit cell so as to create a three-dimensional lattice.
- These spheres are packed into unit cells.
- By considering how atomic spheres are arranged relative to one another, their coordination numbers, and the dimensions of the unit cell, it is possible to form a general view of the structure and complexity of particular crystal structures.
- Notice that this unit cell contains several atoms.
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- Closest packing refers to the most efficient way to arrange atoms in a crystalline unit cells.
- A crystalline material's structure is typically visualized as being composed of unit cells.
- In a three dimensional representation of this hypothetical unit cell—with the spheres packed as efficiently as possible—there are two methods to densely pack the cell.
- Imagine a single layer of spheres packed into the bottom of a unit cell.
- A CCP arrangement has a total of 4 spheres per unit cell and an HCP arrangement has 8 spheres per unit cell.
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- The amount of chemical change that occurs in electrolysis is stoichiometrically related to the amount of electrons that pass through the cell.
- The extent of chemical change that occurs in an electrolytic cell is stoichiometrically related to the number of moles of electrons that pass through the cell.
- It takes 96,485 coulombs to constitute a mole of electrons, a unit known as the faraday (F).
- The "chemistry" in these problems is usually very elementary; the major difficulties usually stem from unfamiliarity with the basic electrical units:
- What mass of copper will be deposited if a current of 0.22 amp flows through the cell for 1.5 hours?
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- Each chemical species has an electrochemical potential (quantity with units of energy) at any given location, which represents how easy or difficult it is to add more of that species to that location.
- The standard cell potential for the reaction is then +0.34 V - (-0.76 V) = +1.10 V.
- The cell equilibrium constant, K, can be derived from the Nernst equation:
- Schematic of a galvanic cell for the reaction between Zn and Cu.
- Calculate the equilibrium constant, K, for a galvanic cell using the Nernst equation
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- These units are called base units, and other units are derived units.
- Different systems of units are based on different choices of base units.
- There are seven SI base units, and all other SI units can be derived from these base units.
- The seven base SI units are: [Physical Quantity: unit symbol (unit name)]
- Derived units are based on units from the SI system of units.
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- The polymeric structure of DNA may be described in terms of monomeric units of increasing complexity.
- Finally, a 5'- monophosphate ester, called a nucleotide may be drawn as a single monomer unit, shown in the shaded box to the right.
- Even the single DNA molecule from an E. coli bacterium is found to have roughly a million nucleotide units in a polymer strand, and would reach a millimeter in length if stretched out.
- Furthermore, accurate copies of the DNA code must be created as cells divide, with the replicated DNA molecules passed on to subsequent cell generations, as well as to progeny of the organism.
- The high molecular weight nucleic acid, DNA, is found chiefly in the nuclei of complex cells, known as eucaryotic cells, or in the nucleoid regions of procaryotic cells, such as bacteria.
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- The units are divided into two classes: base units and derived units.
- Derived units are unlimited in number and are formed by multiplying and dividing the seven base units and other derived units; for example, the SI derived unit of speed is meters per second, m/s.
- Some derived units have special names; for example, the unit of resistance, the ohm (Ω), is uniquely defined by the following relation:
- Another unit of pressure used in meteorology is the bar:
- A prefix may be added to a unit's name to describe a multiple of the original unit.
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- However, an important restriction is that the reagents and conditions must be compatible with the aqueous medium, neutral pH and moderate temperatures found in living cells.
- Nature's solution to carrying out a Claisen-like condensation in a living cell is shown in the bottom equation of the diagram.
- The branched chain and cyclic structures of the terpenes and steroids are constructed by sequential alkylation reactions of unsaturated isopentyl pyrophosphate units.
- In the third diagram, the simplest fashion in which isopentane units combine is termed "head-to-tail".
- Non head-to-tail coupling of isopentane units is also observed, as in the chrysanthemic acid construction shown in the second equation.
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- The basic SI units can be expressed as fractions and multiples of basic units by using a set of simple prefixes.
- The SI system utilizes a standard system of prefixes to the basic units that allow them to be more relevant to and descriptive of relative magnitude.
- Briefly review the basic SI units before you study the prefixes.
- A prefix may be used to identify multiples of the original unit or fractions of the original unit.
- The prefixes redefine the measurement as either a multiple or a fraction of the basic unit.
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- Cell notation is shorthand that expresses a certain reaction in an electrochemical cell.
- Cell notations are a shorthand description of voltaic or galvanic (spontaneous) cells.
- The anode half-cell is described first; the cathode half-cell follows.
- Using these rules, the notation for the cell we put together is:
- A typical arrangement of half-cells linked to form a galvanic cell.