SI units picometer

Based on these data, the atom was believed to consist of a nucleus that accounted Tot most of the mass of the atom, but which occupied only a tiny fraction of its volume. We express atomic (and molecular) dimensions using the SI unit picometer (pm), where... 

Although extremely small—about 10 to 10 meter (m) in diameter— the nucleus nevertheless contains essentially all the mass of the atom. Electrons have negligible mass and circulate around the nucleus at a distance of approximately 10 ° m. Thus, the diameter of a typical atom is about 2 X 10 ° m, or 200 picometers (pm), where 1 pm = 10 m. To give you an idea of how small this is, a thin pencil line is about 3 million carbon atoms wide. Many organic chemists and biochemists still use the unit angstrom (A) to express atomic distances, where 1 A = 100 pm = 10 ° m, but weTl stay with the SI unit picometer in this book. 

Bond distances in organic compounds are usually 1-2 A (1A = 10 °m). Because the angstrom (A) is not an SI unit, we will express bond distances in picometers (1 pm = 10 m). Thus,... 

Bond lengths have usually been, and still often are, measured in angstroms (A) but, with the advent of SI units, the nanometer (10 9 m) and the picometer (10 12 m) are now being used more frequently. In this book we express bond lengths and other molecular dimensions in pi-cometers, which is for many purposes a more convenient unit than the angstrom (1 A = 100 pm). 

We will use angstroms as the unit of measure for bond length because most bond lengths in the chemical literature are still reported in angstroms. The SI unit for bond length is the picometer (pm), where 1 pm = 10" A = 10 m. Thus, 0.958 A = 95.8 pm. 

This unit is used simply because most atomic sizes and chemical bond lengths fall in the range of one to several angstroms, and the use of either picometers or nanometers is slightly awkward. Next, we use the atmosphere (abbreviated atm) as a unit of pressure. It is not a simple power of 10 times the SI unit of the pascal, but rather is defined as follows ... 

As far as possible only SI units have been used in writing equations and presenting experimental data. Angstroms and calories, which still appear in the scientific literature, are avoided. Instead, nanometers and picometers are used for atomic and molecular dimensions, and joules for units of energy. Pressure is discussed in terms of pascals and bars rather than torrs and atmospheres. Equations involving the molecular dipolar properties, namely the dipole moment and polarizability, assume units of coulomb meters and farad square meters, respectively, for these quantities. However, tabulated data are given in the more familiar cgs system with debyes for the dipole moment and cubic nanometers for the polarizability. This follows the usage in most data tabulations at the present time. The connection between the SI and cgs units is explained in chapter 2. The symbols recommended by the International Union of Pure and Applied Chemistry [1] are used as much as possible. 

SI units consist of seven base units and numerous derived units. Exponential notation and prefixes based on powers of 10 are used to express very small and very large numbers. The SI base unit of length is the meter (m). Length units on the atomic scale are the nanometer (nm) and picometer (pm). Volume units are derived from length units the most important volume units are the cubic meter (m ) and the liter... 

Chemists are accustomed to expressing bond lengths in angstrom units, 1 A = 10 ° m. This is not, however, an SI unit. The accepted SI unit for bond length is the picometer (pm) (1 pm = 10 m = 0.01 A). Distances in macromolecular assemblies and biomolecules may be expressed in nanometers... 

Atoms are also extremely small. Most atoms have diameters between 1 X 10 m and 5 X 10 m, or 100-500 pm. A convenient, altoough non-SI, unit of length used to express atomic dimensions is toe angstrom (A). One angstrom equals 10 m. Thus, atoms have diameters on toe order of 1-5 A. The diameter of a chlorine atom, for example, is 200 pm, or 2.0 A. Both picometers and angstroms are commonly used to express toe dimensions of atoms and molecules. 

The distance between nuclei participating in a chemical bond is called the bond length. Every covalent bond has a characteristic bond length. In H— H, it is 74 pm (picometer 1 pm = 10 m). We use SI units of picometers in this book many... 

Although the SI unit for bond length Is the picometer (pm), the angstrom (A) Is still widely used In the chemical literature 1 A = 10" " m. As a result,... 

The angstrom (A) is not a Syst me International (SI) unit. Those who prefer SI units can convert A into picometers (pm) 1 A = 100 pm. Because the angstrom continues to be used by many organic chemists, we will use angstroms in this book. Dipole moment calculations require the bond length to be in centimeters 1 A = 10" cm. 

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