SI Base Units in Chemistry
SI base units are the seven fundamental units of the International System of Units from which all other units are derived; those used most in chemistry are the meter (m), kilogram (kg), second (s), kelvin (K) and mole (mol). In brief: length is measured in meters, mass in kilograms, time in seconds, temperature in kelvin and amount of substance in moles; the liter and the gram are not base units, but they are in daily use in laboratories.
Scientists all over the world use the same system of units, so that measurements can be compared and repeated. The system is called the SI, from the French Système international d’unités. This article lists the seven base units, explains how derived units such as the newton and the joule are built from them, shows the common prefixes, describes the 2019 redefinition and works through unit examples.
What are the seven SI base units?
| Quantity | Unit | Symbol | Used in chemistry for |
|---|---|---|---|
| Length | meter | m | Bond lengths (as nm or pm), glassware dimensions |
| Mass | kilogram | kg | Mass of samples (usually g or mg) |
| Time | second | s | Reaction rates, half-lives |
| Electric current | ampere | A | Electrolysis, batteries |
| Temperature | kelvin | K | Gas laws, thermodynamics |
| Amount of substance | mole | mol | Counting particles in reactions |
| Luminous intensity | candela | cd | Rarely, for light sources |
Which units matter most in chemistry?
Chemists use mass, volume, temperature and amount of substance daily. Some common laboratory units are not SI base units but are accepted for use with the SI:
- Gram (g): 1 g = 0.001 kg. The kilogram is the base unit, but samples in a laboratory are small.
- Liter (L): 1 L = 1 dm3 = 0.001 m3. The cubic meter is the SI unit of volume, and the liter is more practical.
- Degree Celsius (°C): a temperature difference of 1 °C equals 1 K. Absolute temperature T in kelvin is t in °C plus 273.15.
- Mole (mol): the amount of substance that contains 6.02214076 × 1023 elementary entities, such as atoms, molecules or ions.
What are derived units?
Derived units are combinations of base units. Some have special names.
| Quantity | Unit | In base units | Example use |
|---|---|---|---|
| Volume | cubic meter (m3) | m3 | Gas volume |
| Density | kg/m3 (often g/cm3) | kg·m−3 | Identifying substances |
| Force | newton (N) | kg·m·s−2 | Pressure calculations |
| Pressure | pascal (Pa) | kg·m−1·s−2 | Gas laws |
| Energy | joule (J) | kg·m2·s−2 | Heat of reactions |
| Concentration | mol/m3 (often mol/L) | mol·m−3 | Solutions |
Which prefixes are used?
| Prefix | Symbol | Factor | Example |
|---|---|---|---|
| giga | G | 109 | 1 GJ = 1,000,000,000 J |
| mega | M | 106 | 1 MPa = 1,000,000 Pa |
| kilo | k | 103 | 1 kg = 1,000 g |
| centi | c | 10−2 | 1 cm = 0.01 m |
| milli | m | 10−3 | 1 mL = 0.001 L |
| micro | µ | 10−6 | 1 µm = 0.000001 m |
| nano | n | 10−9 | 1 nm = 10−9 m |
| pico | p | 10−12 | 1 pm = 10−12 m |
Worked example: building units
- Density in base units. A density of 2.70 g/cm3 (aluminum). 1 g = 10−3 kg and 1 cm3 = 10−6 m3, so 2.70 × 10−3 ÷ 10−6 = 2.70 × 103 kg/m3, or 2,700 kg/m3.
- Pressure in base units. 1 Pa = 1 N/m2. Since 1 N = 1 kg·m/s2, 1 Pa = 1 kg·m−1·s−2.
- Energy. Lifting a 2.0 kg mass by 0.50 m with g = 9.8 m/s2 requires a force of 2.0 × 9.8 = 19.6 N, and the work is 19.6 × 0.50 = 9.8 J. The joule is 1 N·m = 1 kg·m2·s−2.
- Using prefixes. A bond length of 154 pm is 154 × 10−12 m = 1.54 × 10−10 m = 0.154 nm.
How are SI units defined today?
Since 20 May 2019, all SI base units are defined by fixing exact values of seven natural constants. For example, the meter is defined using the speed of light in a vacuum, 299,792,458 m/s, and the second from a frequency of the cesium-133 atom (9,192,631,770 cycles per second). The kilogram is defined by fixing the Planck constant, and the mole by fixing the Avogadro constant at 6.02214076 × 1023 per mole. Before, the kilogram was defined by a physical object, a platinum-iridium cylinder kept near Paris.
How did the metric system start?
The metric system was created in France in the 1790s after the French Revolution, with the meter based on a fraction of the distance from the North Pole to the equator, and the gram based on the mass of a cubic centimeter of water. An international treaty, the Metre Convention, was signed in 1875, and the modern SI was formally adopted by the General Conference on Weights and Measures in 1960. Today almost every country uses the metric system officially, though the United States still uses customary units in daily life.
Which mistakes are common?
Mistake 1: Treating the gram as the base unit of mass
The kilogram is the base unit, though the gram is more convenient in the laboratory.
Mistake 2: Writing units with the wrong case
A capital K is kelvin, while lower-case k means kilo. mol is mole, and Mol is not correct. The symbol for the second is s, not sec.
Mistake 3: Using a degree sign with kelvin
The unit is written 300 K, not 300 °K.
Mistake 4: Mixing units in a calculation
Convert everything to compatible units before calculating, for example grams with g/mol and liters with mol/L.