# Science Helpdesk

## Matter

#### Key Information

##### 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:

QuantityNameSymbol
Lengthmetrem
Masskilogramkg
Timeseconds
Electric currentampereA
TemperaturekelvinK
Amount of substancemolemol
Luminous intensitycandelacd

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:

 Name giga mega kilo hecto deca deci centi milli micro nano Symbol G M k h da d c m µ n Factor 109 106 103 102 101 10-1 10-2 10-3 10-6 10-9
##### Quantities

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
SymbolAtomic numberAtomic mass% in continental crustRequired for Life
AluminumAl13278.2300No
BromineBr35800.00025No
CalciumCa20404.1000Yes
CarbonC6120.0200Yes
ChlorineCl1735.50.0130No
CobaltCo27590.0025No
CopperCu2963.50.0055Yes
FluorineF9190.0625No
GoldAu791970.0000004No
HydrogenH111.4000Yes
IodineI531270.00005No
IronFe26565.6000Yes
LithiumLi360.0020No
MagnesiumMg12242.3000Yes
ManganeseMn25550.0950Yes
MercuryHg802010.000008No
MolybdenumMo42960.00015Yes
NickelNi28590.0075No
NitrogenN7140.0020Yes
OxygenO81646.4000Yes
PhosphorusP15310.1050Yes
PotassiumK19392.1000Yes
SiliconSi142828.2000No
SilverAg471080.000007No
SodiumNa11232.4000No
SulphurS16320.0260Yes
TinSn501190.00020No
UraniumU922380.00027No
ZincZn30650.0070Yes
[Source]