The "Metric" system 

    Called the "metric" system, based on the French word for measure. The driving force was the growing importance of weights in the sciences, especially chemistry. At that time, every country had their own system of weights and measures. England had three different systems just within its own borders!!

On May 20, 1875, delegates of 17 countries signed the Meter Convention. It was amended in 1921 and today 48 countries are signatories.

The modern metric system has been renamed Systeme International d'Unites (International System of Units) and is denoted by the letters SI. SI was established in 1960, at the 11th General Conference on Weights and Measures. It was then that units, definitions, and symbols were revised and simplified.

Definition provided by Chemteam 

Provided by  Marcello Da Cunha

Base Units 

SI units are divided into two classes, base units and derived units. The base units are dimensionally independent and the derived units are formed by combinations of powers of the base units. Simple put, base units are the foundational units of the universe, from which everything in built on. 

Meter (m) 

Measurement of length  

The meter is the length of the path traveled by light in a Vacuum during a time interval of 1/299792458 of a second.

The width of hydrogen atom is 1x10^-9 m while Alpha Centauri (the closest star to the sun) is 3.8 x10^16 meters away

 Second (s)

Measurement of time 

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

A health adult has one heart beat ever second while the average  lifespan of a star is over 1x10^13 seconds

 Kilogram (kg)

Measurement of mass

 The Kilogram is the unit of mass; it is equal to the mass of the international prototype of the kilogram.  Of all of the bass units the kilogram is the only one based on an artifact

 The mass of a single hydrogen atom is 1.8x10^-36 kg while our sun has a mass of 6.2x10^11

Kelvin (K)

Measurement of Temperature 

 The Kelvin, unit of thermodynamic temperature, is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water. Units of Kelvin and Amperes are both capitalized because both units are named after a person working in the field.

The coldest recorded temperature was 3 Kelvin (even though we have produced temperature less then that in last couple of years but it has been primarily for a few atoms) while the surface of our sun is 5778 K 

Ampere or Amp (A) 

Measurement of electrical current

 The ampere is that constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular cross-section, and placed 1 meter apart in vacuum, would produce between these conductors a force equal to 2x10^−7 newton per meter of length.

A typical household circuit has 15 amps following through it while the Large Hadron collider  at CERN12,000 amps of current flows through its magnets

Mole (mol)

Measurement of the number of particles in a substance

 The mole is the amount of substance of a system which contains as many elementary entities as there are atoms in 0.012 kilogram of carbon 12; its symbol is mol.

Candela (cd)

 Measurement of the luminous intensity

 The candela is the luminous intensity, in a given direction, of a source that emits monochromatic radiation of frequency 540x10^12 hertz and that has a radiant intensity in that direction of 1/683 watt per steradian.

 RANT ALERT      RANT ALERT 

        When scientist peeled some of the mysteries of the universe, our understanding of how things work changed, some of these base units seem to be less base and more derived.  Take current for example, current is the measure of how much charge that travels though an area, or coulomb (charge) per second (time), so it seems that the base unit should be a coulomb and not Ampere, but because we discovered current before the charge, the Ampere is the base unit.  The same could be said for temperature, moles and candelas, and even mass when we look at nuclear physics doesn't seem like a base unit.  The argument can be made that our base units should be length, time, charge, and energy, but because of human nature our base units have been and always will be those fundamental 7.  (think of it this way, remember the stink that was created when Pluto was striped of it's planet-hood? now think of the stink that would be created with scientist if you striped moles or kilograms from the list of building blocks of the universe 

Derived units

 The international System of Units (SI) specifies a set of seven base units from which all other units of measurement are formed. These other units are called SI derived units and are also considered part of the standard. SI units was after the French Le Système International d'Unités which opted for a universal, unified and self-consistent system of measurement units based on the MKS (metre-kilogram-second) system.

Hertz (Hz) 

A unit of Frequency 

Expressed in base units 1/s 

s^-1 

Newton (N) 

A unit of force 

Expressed in base units kg m/s/s 

kg m s^-2 

Coulomb (C) 

A unit of electrical Charge 

Expressed in base units A s 

 A s 

Joules (J) 

A unit of energy  

Expressed in base units kg m^2/s/s 

kg m^2 s^-2 

And many others