Units

In the International System of units (SI, from the French Système International d’Unités), the units are formed from seven base units listed in Table A1.1. All other physical quantities may be expressed as combinations of these physical quantities and reported in terms of derived units. Thus, volume is (length)3 and may be reported as a multiple of 1 metre cubed (1 m3), and density, which is mass/volume, may be reported as a multiple of 1 kilogram per metre cubed (1 kg m⁻³). A number of derived units have special names and symbols. The names of units derived from names of people are lower case (as in torr, joule, pascal, and kelvin), but their symbols are upper case (as in Torr, J, Pa, and K). Among the most important for our purposes are those listed in Table A1.2. In all cases (both for base and derived quantities), the units may be modified by a prefix that denotes a factor of a power of 10. In a perfect world, Greek prefixes of units are upright (as in µm) and sloping for physical properties (as in µ for chemical potential), but available typefaces are not always so obliging. Among the most common prefixes are those listed in Table A1.3. Examples of the use of these prefixes are

1 nm = 10⁻⁹ m 1 ps = 10⁻¹² s 1 µmol=10⁻⁶mol

The kilogram (kg) is anomalous: although it is a base unit, it is interpreted as 103 g, and prefixes are attached to the gram (as in 1 mg = 10⁻³ g). Powers of units apply to the prefix as well as the unit they modify:

1 cm³ = 1 (cm)³ = 1 (10⁻² m)³ = 10⁻⁶ m³

Note that 1 cm3 does not mean 1 c(m3). When carrying out numerical calculations, it is usually safest to write out the numerical value of an observable as powers of 10. There are a number of units that are in wide use but are not a part of the International System. Some are exactly equal to multiples of SI units. These include them litre (L), which is exactly 103 cm3 (or 1 dm3) and the atmosphere (atm), which is exactly 101.325 kPa. Others rely on the values of fundamental constants, and hence are liable to change when the values of the fundamental constants are modified by more accurate or more precise measurements. Thus, the size of the energy unit electronvolt (eV), the energy acquired by an electron that is accelerated through a potential difference of exactly 1 V, depends on the value of the charge of the electron, and the present (2005) conversion factor is 1 eV = 1.602 177 33 × 10⁻¹⁹ J. Table A1.4 gives the conversion factors for a number of these convenient units.

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مواضيع ذات صلة

Names of quantities

Working galvanic cells

Electrolysis

Experimental techniques

The rate laws

The electric potential at the interface

The structure of the interface

Catalytic activity at surfaces

Mechanisms of heterogeneous catalysis

Mobility on surfaces

The rate of desorption

The rate of adsorption