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topic 004

The International System of Units

The International System of Units or Systeme Internationale (SI) is an improved metric system adopted by the Eleventh General Conference of Weights and Measures in 1960. It is the universal measuring system used in all areas of science throughout the world. The entire SI system of measurement is constructed from seven base units, each of which represents a single physical quantity as shown in the table below. For our chemistry class, we shall only consider five of these units.
Base Units of the International System 
Quantity                    Name of Unit          Unit Symbol

length                      meter (metre)          m
mass                        kilogram               kg
time                        second                 s
temperature                 kelvin                 K (case sensitive)
amount of substance         mole                   mol
Like earlier versions of the metric system, the SI units can be designated as decimal fractions or multiples by the use of appropriate prefixes. The acceptable SI prefixes are given in the table below. 

Prefixes of the International System

Factor        Prefix        Symbol        Factor        Prefix        Symbol

                                          10e-15         femto         f
10e12         tera          T             10e-12         pico          p
10e9          giga          G             10e-9          nano          n
10e6          mega          M             10e-6          micro()         
10e3          kilo          k             10e-3          milli         m
                                          10e-2          centi         c
These prefixes are of critical importance, you must take the time to become completelyl familiar and comfortable with them!

N.B.  Whenever exponents are used with SI prefixes on either base units or derived units, the exponent applies to the prefix as well as to the unit. For example, nm2, or square nanometer, is interpreted as (nm)2 rather than n(m2). Any prefix can be applied to any base unit except the kilogram; the kilogram takes prefixes as if the base unit were the gram. As a consequence 10-6 kg is written as 1 milligram (mg) rather than 1 microkilogram.


The great advantage of the SI over other systems of units is that when any physical quantity whatever is written out in the SI base units or in units derived only from the SI base units, any mathematical manipulations performed with them will follow the quantity calculus. No conversion factors will ever be required. This means that if the symbols in any equation are replaced by real numbers with their SI base units and algebraic manipulations are performed upon the units in exactly the same way as they are performed upon the numbers to which those units refer, the result will come out with the correct numbers and units.

Example. The mass of a sample of pure rhombic sulfur was 150.637 g and the volume of water it displaced was 72.8 mL. The density of sulfur is then (150.637 g)/(72.8 mL) = 2.07 g/mL, or g/cm3, or kg/dm3, or kg/L.

Base Units of the SI
LENGTH
The SI unit of length is the meter, a fundamental unit of the SI. The meter was once defined in terms of the circumference of the earth as part of the older metric system. Since 1983 the meter is by definition the length of the path travelled by light in vacuum in 1/299,792,458 of a second. ,
Some units of length worth memorizing:  one inch =25.4 mm exactly, meter, centimeter, millimeter, micrometer (micron), nanometer and the Angstrom (derived unit).

You need to be familiar with the km, m, cm, mm, m or micron, and a nanometer.  Also, 10 nm are equal to 1 angstrom (A).

MASS
The SI unit of mass is the kilogram, a fundamental unit of the SI. The kilogram was once defined as the mass of one cubic decimetre (L) of water. Since 1901 it is by definition the mass of the international prototype of the kilogram, a platinum-iridium mass which is stored at Sevres in France.
An interesting fact about the kilogram is that it is the only SI base unit to incorporate a prefix. The kilogram is the only SI unit based on a finite amount of material rather than some physical parameter.

You need to be familar with the kg, g, mg, g and ng

TIME
The SI unit of time is the second, a fundamental unit of the SI. Originally defined in terms of the rotation of the earth, the second is now defined in terms of atomic transitions in 133Cesium because these are subject to more precise measurement. Specifically, since 1967 the second is defined as the duration of 9,192,631,770 periods of the electromagnetic radiation corresponding to the transition between the two hyperfine levels of the ground state of the 133Cs atom.
TEMPERATURE
The SI unit of temperature is the kelvin, a fundamental unit of the SI. Since 1967, the kelvin has been by definition the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. The triple point of water is the temperature at which ice, water, and water vapor can all exist in equilibrium and its value is +0.01o Celsius.

The kelvin (which is correctly written without a degree sign) is used for measuring both temperature and temperature interval; thus one can say, "The temperature is 300 K" or "This pan is 20 K hotter than that one." Temperatures in kelvin can only be positive and so they require no sign. The kelvin scale of temperature is also known as the absolute scale and the thermodynamic scale. Conversion factors between temperatures in degrees Celsius (oC) and temperatures in kelvin are:
 
Temperature (oC) + 273.15 (exactly) = temperature (K) (memorize this)
N.B. The degree Celsius, the unit of the common metric temperature scale, is not part of the SI but its use is not discouraged. A temperature interval in degrees Celsius is identical to a temperature interval in kelvin, although a temperature in degrees Celsius is not identical to a temperature in kelvin.

AMOUNT OF SUBSTANCE
The SI unit of quantity or amount of substance is the mole, a fundamental unit of the SI. There are no other modern units in which amount of substance is measured, so no conversion factors are required. Often, however, units of mass or volume are used to give the amount of substance. Conversion of these to the mole requires the use of appropriate measured physical constants, the molar mass or the molar volume. Since 1971, by definition one mole of entities is the same number of entities as there are atoms of carbon12 in exactly 0.012 kilogram of carbon-12, which is Avogadro's number of entities (approximately 6.0221 x 1023 entities).
You need to be familiar with a mol, mmol, mol, nmol and a pmol.

Non-SI Units Commonly Used (YOU NEED TO MEMORIZE THESE)

1) Liter: symbol = L.

2) cubic centimeter: symbol = cm3. Often used for measuring the volume of solids, one cm3 equals
one milliliter (mL). The mL has emerged as the common measure of volume.

3) Ångström: symbol = Å. One Å equals 10¯8 cm.  THERE ARE 10Å IN ONE NANOMETER.