The International System of Units[Source: Wikipedia]
The International System of Units (abbreviated SI from the French language name Système international d'unités) is the world's most widely used system of units. The SI was developed in 1960 and, with few exceptions, is used in every country in the world.
The SI base units for the seven primary quantities are:
|Amount of substance||mole||mol|
Symbols are written in lower case, except for symbols derived from the name of a person. For example, the unit of electric current is named after André-Marie Ampère, so its symbol is written "A", whereas the unit itself is written "ampere". The only exception is the litre, whose original symbol "l" is unsuitably similar to the numeral "1"; thus it is recommended that "L" be used instead.
Abbreviated symbols should not be pluralized: for example "25 kg", not "25 kgs".
Symbols do not have an appended period (.) unless at the end of a sentence.
A prefix may be added to units to produce a multiple of the original unit. All multiples are integer powers of ten. For example, kilo- denotes a multiple of a thousand and milli- denotes a multiple of a thousandth. The SI main prefixes are as follows:
Quantities are the measurable properties of the physical bodies. Some of the most important ones are the following:
|Mass||Is the amount of matter in a body. Its unit in the SI is the kg.|
|Volume||Tells how much space an object occupies. Its unit in the SI is the m3.|
|Capacity||Is the amount of space that can be contained by a body. It is measured in L.|
|Density||Expresses how concentrated is the matter in a body, this is, how much matter is there in a given unit of volume in a body. It is always the same for the same type of substance, this is, it doesn't depend on the size of the object. Its unit in the SI is the kg/m3.|
|Temperature||It is not a kind of energy but a measure of the amount of heat in a body. It depends on the movement of its particles: the quicker the movement, the higher the temperature. Its unit in the SI is the K, but is commonly expressed in °C or °F.|
Characteristics of Some of the Common Chemical Elements Found in the Earth's Crust
|Symbol||Atomic number||Atomic mass||% in continental crust||Required for Life|