The SI baseIn Arrhenius theory, a substance that increases the concentration of hydroxide ions in an aqueous solution. In Bronsted-Lowry theory, a hydrogen-ion (proton) acceptor. In Lewis theory, a species that donates a pair of electrons to form a covalent bond. units are not always of convenient size for a particular measurement. For example, the meterThe SI unit for distance or length. would be too big for reporting the thickness of this page, but rather small for the distance from Chicago to Detroit. To overcome this obstacle the SI includes a series of prefixes, each of which represents a power of 10. These allow us to reduce or enlarge the SI base units to convenient sizes. The figures below show how these prefixes can be applied to the meter to cover almost the entire range of lengths we might wish to measure.
Prefixes Used for Decimal Fractions and Multiples of SI UnitsThe international system of units (Système International d'Unité) based on seven fundamental units: meter, kilogram, second, ampere, kelvin, candela, mole..
One non-SI unit of length, the angstromA unit of length equal to 100 pm or 10-10 m. (Å), is convenient for chemists and will continue to be used for a limited time. Since 1Å = 10–10 m, the angstrom corresponds roughly to the diameters of atomsThe smallest particle of an element that can be involved in chemical combination with another element; an atom consists of protons and neutrons in a tiny, very dense nucleus, surrounded by electrons, which occupy most of its volume. and small molecules. Such dimensions are also conveniently expressed in picometers, 1 pm = 10–12 m = 0.01Å, but the angstrom is widely used and very familiar. Therefore we will usually write atomic and molecular dimensions in both angstroms and picometers.
The SI base unit of massA measure of the force required to impart unit acceleration to an object; mass is proportional to chemical amount, which represents the quantity of matter in an object., the kilogramThe SI unit for mass., is unusual because it already contains a prefix. The standard kilogram is a cylinder of corrosionChemical or electrochemical oxidation of the surface of a metal or alloy; corrosion can cause structural damage, as in the case of rusting.-resistant platinum-iridium alloyA solid that has metallic properties and is made up of two or more elements. which is kept at the International Bureau of Weights and Measures near Paris. The kilogram was chosen instead of a gram because the latter would have made an inconveniently small piece of platinum-iridium and would have been difficult to handle. Also, units of force, pressureForce per unit area; in gases arising from the force exerted by collisions of gas molecules with the wall of the container., energyA system's capacity to do work., and power have been derived using the kilogram instead of the gram.
Despite the fact that the kilogram is the SI unit of mass, the standard prefixes are applied to the gram when larger or smaller mass units are needed. For example, the quantity 106 kg (1 million kilograms) can be written as 1 Gg (gigagram) but not as 1 Mkg (megakilogram). The operative rule here is that one and only one prefix should be attached to the name for a unit. Figure 1.6 illustrates the use of this rule in expressing the wide range of masses available in the universe. Note that the masses of atoms and molecules are usually so small that scientific notation must be used instead of prefixes.