CoreChem:SI Prefixes
From ChemEd Collaborative
{reg}1.3.2 SI Prefixes
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- What Chemists Do
- Measurement
- Numbers, Units, Quantities
- Handling Large and Small Numbers with Units
- The International System of Units (SI)
- SI Prefixes
- Volume
- Density
- Conversion Factors
- Impact
Atoms, Molecules and Chemical Reactions (Original)
- Macroscopic Properties and Microscopic Models
- Historical Development of the Atomic Theory
- The Atomic Theory
- Macroscopic and Microscopic Views of a Chemical Reaction
- Testing the Atomic Theory
- Chapter 6 - CHEMICAL BONDING—ELECTRON PAIRS AND OCTETS
- 6.1 Ionic Bonding
- 6.2 Periodic Variation of ionization Energy and Electron Affinity
- 6.3 Binary Ionic Compounds and Their Properties
- 6.4 The Covalent Bond
- 6.5 Covalent Molecules and the Octet Rule
- 6.6 Writing Lewis Structures for Molecules
- 6.7 Ionic Compounds Containing Polyatomic Ions
- 6.8 The Sizes of Atoms and Ions
- Summary 6
16.1 Spontaneous Processes and Molecular Probability
16.2 Thermodynamic Probability and Entropy
16.3 Getting Acquainted with Entropy
16.4 Including the Surroundings
16.5 The Free Energy
16.6 Equilibrium Constants Revisited
Summary 16
17.1 Electrolysis
17.2 Commercial Applications of Electrolysis
17.3 Quantitative Aspects of Electrolysis
17.4 Galvanic Cells
17.5 Electromotive Force of Galvanic Cells
17.6 Commercial Galvanic Cells
17.7 Galvanic Cells and Free Energy
Summary 17
18.1 Experimental Measurement of Rates
18.2 Microscopic View of Chemical Reactions
18.3 Reaction Mechanisms
18.4 Increasing the Rate of a Reaction
18.5 Some Important Types of Catalysts
Summary 18
19.1 Naturally Occurring Nuclear Reactions
19.2 Artificially Induced Nuclear Reactions
19.3 Nuclear Stability
19.4 The Rate of Radioactive Decay
19.5 Detection and Measurement of Radiation
19.6 Uses of Artificial Isotopes in Chemistry
19.7 Mass-Energy Relationships
19.8 Nuclear Fission
19.9 Nuclear Fusion
Summary 19
20.1 The Elements of Life
20.2 The Building Blocks of Biochemistry
20.3 Fats and Lipids
20.4 Carbohydrates
20.5 Proteins
20.6 Nucleic Acids
Summary 20
21.1 The Nature of Electromagnetic Radiation
21.2 Atomic Spectra and the Bohr Theory
21.3 The Spectra of Molecules: Infrared
21.4 The Visible and Ultraviolet Spectra of Molecules: Molecular Orbitals
Summary 21
22.1 Metallic Bonding
22.2 Metallurgy
22.3 Coordination Compounds
22.4 Transitional Metal Ions in Aqueous Solutions
Summary 22
{/reg} The SI base units are not always of convenient size for a particular measurement. For example, the meter 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 (Table 1.3). These allow us to reduce or enlarge the SI base units to convenient sizes. Figure 1.5 shows how these prefixes can be applied to the meter to cover almost the entire range of lengths we might wish to measure.
TABLE 1.3 Prefixes Used for Decimal Fractions and Multiples of SI Units.
One non-SI unit of length, the angstrom (Å), is convenient for chemists and will continue to be used for a limited time. Since 1Å = 10–10 m (see Fig. 1.5), the angstrom corresponds roughly to the diameters of atoms 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 mass, the kilogram, is unusual because it already contains a prefix. The standard kilogram is a cylinder of corrosion-resistant platinum-iridium alloy 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, pressure, energy, 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.
