SI Units - a summary

Skill to develop

Know (not memorize) the seven (7) basic quantities and their SI Units.
Explain what the basic units are.
Explain what are derived units, and how they are derived.
Represent all quantities with proper numbers and units whenever possible.

List what all the basic quantities and their units you know of and search for those that you do not know yet. When you feel confident about your leaning, You can practice writing a CACT Quiz on measurements and units over the Internet. The quiz mark will not be counted for your quiz average.

SI Units

The following review is not intended to replace your text, but it gives the major points about SI units. Since all scientific measurements involve quantities and units, the SI units is important in your expression of physical and chemical measurements.

Understanding and proper expression of quantities are basic skills for any modern educated person. You have to master all quantities described here. Otherwise, you may not be able to understand or follow the rest of these web pages and lectures.

If you find the information too difficult to remember, you probably have not got a solid background in physics and chemistry. Since you cannot start over again, the best way is to realize that you have to represent quantities. Each time you are dealing with a quantity, analyze it and think through what it really means. Slowly, you will build your foundation. Understand a few fundamental things well is better than memorize a lot of things with no comprehension.

The Basic Units

Quantities form the basis for science and engineering and any moment of our lives. Unless you have expressed the quantities in numbers and units, you have not expressed anything. Quantities are defined only when they are expressed in numbers and sunits. Missing units and improper use of units are serious omissions and errors.

Formats of the symbols used here may be slightly different from those in textbooks due to the limitation of computer representations.

Years ago, physicists used either the mks (meter-kilogram-second) system of units or the cgs (centimeter-gram-second) systems for length, mass, and time. In addition to these three basic quantities are four others: the electric charge or current, temperature, luminous intensity and the amount of substance. Chemical quantities are mostly based on the last one. Thus, these are seven basic quantities, each has an unit.

The international system of units (Systeme International d'Units) was adopted by the General Conference on Weights and Measure in 1960, and the SI units are widely used today. All SI units are based on these basic units.

Seven Basic Quantities and Units
Quantity	Unit	Symbol

Close your eyes, and see if you can name the 7 fundamental quantities in science and their (SI) Units. Science is based on only 7 basic quantities, for each, we have to define a standard unit. Think why these are the basic quantities. Are these related to any other quantities? Can they be derived from other quantities?

Derived Units

There are other quantities aside from the seven basic quantities mentioned above. However, all other quantities are related to the basic quantities. Thus, their units can be derived from the seven SI units above. For this reason, other units are called derived units The table below lists some examples:

Derived quantities and their SI units
Quantity	Unit	Symbol

Derived units can be expressed in terms of basic quantities. From the specific derived unit, you can reason its relationship with the basic quantities.

For some specific common quantities, the SI units have special symbols. As you use these often, you will feel at home with them. To remember it is very hard. However, you wil encounter them during your study of these quantities. The are collected here to point out to you that these are speical SI symbols.

Special symbols of some SI units
Quantity	Unit	Explanation

Common Units still in Use

The following units are still in common use for chemistry. There are some other commonly used units too, but their meaning are clear by the time you use them.

Common units still in Use
Quantity	Symbol	Explanation

* The use of m for molality and for meter is sometimes confusing.

Units for radiation:

The following units are used in special technologies or disciplines. Since most people are not familiar with them, they are explained in more details here.

Becquerel: the SI unit for radioactivity symbol (B), which is 1 disintegration per second (dps). 1 Ci = 3.7e10 B.
Curie (Ci): a unit of radioactivity originally based on the disintegration rate of 1 g of radium. Now a Curie is the quantity of radioactive material that has a disintegration rate of 3.700e10 per second (B). 1 mCi = 1e-3 Ci; 1 microCi = 1e-6 Ci; 1 MCi = 1e6 Ci.
Gray and Rad: radiation dose units. The gray (Gy) is an SI unit for the absorption of 1 J radiation energy by one kg of material. The rad was a popular units, which is the absorption of 100 erg of radiation energy by one gram, (1 Gy = 100 rad).
Roentgen (R): a unit for the measure of X-ray and gamma ray exposure. When 1 R = 93 erg per g (1 R = 0.93 rad for X-rays or gamma rays whose energy is above 50 keV).

The unit erg is for energy, 1 J = 10,000,000 erg.