We generally deal with a large number of physical quantities. Each physical quantity requires a separate unit for its measurement.

As a result, we have to deal with a large number of units of different kinds. It is not practical to deal with as many units as there are physical quantities.

With the advances in science, the need of a uniform system of measurements was felt. To make measurements more scientific, convenient and uniform, the French Academy of Science in 1791 devised a metric system of measurement based on decimal system.

In October 1960, the XIth General Conference of Weights and Measures adopted an International Systems of Units. The units in this system are known as SI Units. The abbreviation SI comes from French word, Systeme Internationale d’ Unites.

The SI units are now accepted universally for use in the scientific literature.

Table of Contents

## Base SI units

There are seven base units in SI system

Quantity | Symbol for quantity | Name of SI unit | Symbol for SI unit |
---|---|---|---|

Length | l | metre | m |

Mass | m | kilogram | kg |

Time | t | second | s |

Electric Current | I | ampere | A |

Thermodynamic Temperature | T | kelvin | K |

Amount of substance | n | mole | mol |

Luminous Intensity | I_{v} | candela | cd |

Below are the definitions of seven base SI units.

1. Length – Base unit of length or distance is **metre (m)**.

A metre is defined as distance travelled by light in vacuum during 1/299 792 458 second

2. Mass – Unit of mass is **kilogram (kg)**

A kilogram is defined as mass of platinum-iridium cylinder (known as the standard kilogram) kept at the International Bureau of Weights and Measures at Sevres, France.

3. Time – Unit of time is **second**

A second is defined as the duration of 9,192,631,770 periods of the radiation corresponding to the transition between two hyperfine levels of the ground state of the cesium-133 atom.

4. Temperature – Unit of temperature is **kelvin(K)**

The thermodynamic scale on which temperature is measured has its zero at absolute zero, and has a fixed point corresponding to 273.15 K at the triple point of water.

5. Electric Current – Unit of electric current is **ampere (A)**

Ampere is defined as the magnitude of the current that when flowing through each of the two parallel wires of length equal to 1 m, separated by 1 metre in free space, results in a force of

2 × 10^{-7} N between the two wires.

6. Amount of substance – Unit of amount of substance is **mole**

A mole is defined as the amount of any substance which contains as many elementary units as there are atoms in exactly 0.012 kg (12 g) of ^{12}C carbon atoms.

7. Luminous intensity – Base unit of luminosity is **candela**

Candela is defined as luminous intensity of 1/600000 of a square metre of a radiating cavity at the freezing temperature of platinum (2042K).

## Rules for writing SI Units

1. While writing a unit, only singular form is used.

For example – the mass of any substance should be written as kg not as kgs. Distance should be expressed in km not in kms.

So writing 10 kg and 100 m is correct

But 10 kgs and 100 ms is in correct

2. Abbreviation for any unit must not have a full stop at the end, unless it’s appearing at the end of sentence.

So writing – **My weight is 10 kg.** correct

But writing – **My cousin’s weight is 10 kg. and still she eats a lot.** Incorrect

3. One space is left between the last digit of a numeral and the symbol of the unit.

So writing – 10m is incorrect

But writing – 10 m is correct

4. Words and symbols for units should not be mixed together.

So writing – **10 kg per cubic metres** is incorrect

But writing – **10 kg/m ^{3}** or

**10 kg per cubic metre**is correct

5. With numerals symbol of unit should be written.

So writing –

**100 cm**is correct

But writing –

**one hundred cm**is incorrect

6. The names of units derived from people’s names are written in small letters.

For example – unit named after Joule is written as joule (J).

Unit named after Newton is written as newton (N).

7. Sign of degree(°) is not written when Kelvin scale is employed.

For example – temperature is written as 273 K not as 273°K

## Prefixes in SI Units

SI units are used for expressing quite small and very large measurements but writing numbers like 0.0000000183 m or 18978983748932 kg is not easily readable.

That’s why people use prefixed of 10 to power to express both small and quite large measurements.

Small or large can be written as Number × 10^{Prefix}

Below is table showing common prefixes used in Chemistry.

Multiple | Prefix | Symbol |
---|---|---|

10^{18} | exa | E |

10^{15} | peta | P |

10^{12} | tera | T |

10^{9} | giga | G |

10^{6} | mega | M |

10^{3} | kilo | k |

10^{2} | hacto | h |

10^{1} | deka | da |

10^{-1} | deci | d |

10^{-2} | centi | c |

10^{-3} | milli | m |

10^{-6} | micro | μ |

10^{-9} | nano | n |

10^{-12} | pico | p |

10^{-15} | femto | f |

10^{-18} | atto | a |

## Advantages of SI Units

Advantage | Description |
---|---|

Coherent System of units | All derived SI units can be obtained by dividing and multiplying base SI units and no numerical factors are involved as used to be the case for other systems of units (C.G.S. and M.K.S. systems) |

Rational System of units | In SI system of units, each unique physical quantity have unique unit. For example – all types of energies like Mechanical energy, Heat energy etc. all have only one single unit joule (J) |