Standard Units (SI Units)
The International System of Units (abbreviated SI) is the metric system used in science, industry, and medicine.
Learning Objective

Recognize SI units and their importance for measurment
Key Points
 Every field of science involves taking measurements, understanding them, and communicating them to others. In other words, we all have to speak the same basic language.
 The SI system, also called the metric system, is used around the world.
 There are seven basic units in the SI system: the meter (m), the kilogram (kg), the second (s), the kelvin (K), the ampere (A), the mole (mol), and the candela (cd).
Term

SI system
A series of units that is accepted and used throughout the scientific world.
Full Text
The Need for a Common Language
Every field of science involves taking measurements, understanding them, and communicating them to others. In other words, we all have to speak the same basic language. Whether you are a chemist, a physicist, a biologist, an engineer, or even a medical doctor, you need a consistent way of communicating size, mass, shape, temperature, time, amount, energy, power, and speed.
Consider the screen on which you're reading this text right now. It might be an LCD screen, which is made up of liquid crystals. The chemist developing a specific formulation for a liquid crystal has to meaningfully communicate information to an engineer so that the engineer knows how to manufacture it. The engineer, in turn has to be able to communicate with other engineers, physicists, and chemists to design the circuit boards, display screens, and electronic interfaces of the rest of the computer. If these people don't all speak the same language, the enterprise will never get off the ground.
The International System of Units (abbreviated SI, from the French Système international d'unités) is the metric system used in science, industry, and medicine. Depending on your age and geographic location, you might be very familiar with the "imperial" system, which includes units such as gallons, feet, miles, and pounds. The imperial system is used for "everyday" measurements in a few places, such as the United States. But in most of the world (including Europe) and in all scientific circles, the SI system is in common use.
Units of the SI System
There are seven base units in the SI system:
 the kilogram (kg), for mass
 the second (s), for time
 the kelvin (K), for temperature
 the ampere (A), for electric current
 the mole (mol), for the amount of a substance
 the candela (cd), for luminous intensity
 the meter (m), for distance
The Seven SI Units
This figure displays the fundamental SI units and the combinations that lead to more complex units of measurement.
It should be apparent that the move into modern times has greatly refined the conditions of measurement for each basic unit in the SI system, making the measurement of, for example, the luminous intensity of a light source a standard measurement in every laboratory in the world. A light source made to produce 20 cd will be the same regardless of whether it is made in the United States, in the UK, or anywhere else. The use of the SI system provides all scientists and engineers with a common language of measurement.
History of the SI System
The SI units of measurement have an interesting history. Over time they have been refined for clarity and simplicity.
 The meter (m), or metre, was originally defined as 1/10,000,000 of the distance from the Earth's equator to the North Pole measured on the circumference through Paris. In modern terms, it is defined as the distance traveled by light in a vacuum over a time interval of 1/299,792,458 of a second.
 The kilogram (kg) was originally defined as the mass of a liter (i.e., of one thousandth of a cubic meter). It is currently defined as the mass of a platinumiridium kilogram sample maintained by the Bureau International des Poids et Mesures in Sevres, France.
 The second (s) was originally based on a "standard day" of 24 hours, with each hour divided in 60 minutes and each minute divided in 60 seconds. However, we now know that a complete rotation of the Earth actually takes 23 hours, 56 minutes, and 4.1 seconds. Therefore, a second is now defined as 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 cesium133 atom.
 The ampere (A) is a measure of the amount of electric charge passing a point in an electric circuit per unit time. 6.241×10^{18} electrons, or one coulomb, per second constitutes one ampere.
 The kelvin (K) is the unit of the thermodynamic temperature scale. This scale starts at 0 K. The incremental size of the kelvin is the same as that of the degree on the Celsius (also called centigrade) scale. The kelvin is the fraction 1/273.16 of the thermodynamic temperature of the triple point of water (exactly 0.01 °C, or 32.018 °F).
 The mole (mol) is a number that relates molecular or atomic mass to a constant number of particles. It is defined as the amount of a substance that contains as many elementary entities as there are atoms in 0.012 kg of carbon12.
 The candela (cd) was so named to refer to "candlepower" back in the days when candles were the most common source of illumination (because so many people used candles, their properties were standardized). Now, with the prevalence of incandescent and fluorescent light sources, the candela is defined as the luminous intensity in a given direction of a source that emits monochromatic radiation of frequency
$540 \cdot 10^{12}$ Hertz and that has a radiant intensity in that direction of 1/683 watts per steradian.
Key Term Reference
 Celsius
 Appears in these related concepts: The Structure and Properties of Water, Heating Curve for Water, and Hydrogenation
 SI unit
 Appears in these related concepts: Using Molarity in Calculations of Solutions, SI Units of Pressure, and Molarity
 Triple point
 Appears in these related concepts: Kelvin Scale, Major Features of a Phase Diagram, and Solid to Gas Phase Transition
 atom
 Appears in these related concepts: Description of the Hydrogen Atom, Early Ideas about Atoms, and Stable Isotopes
 atomic mass
 Appears in these related concepts: Balancing Nuclear Equations, Atomic Number and Mass Number, and Average Atomic Mass
 base
 Appears in these related concepts: Temple Architecture in the Greek Orientalizing Period, Rules for Exponent Arithmetic, and Rational Exponents
 coulombs
 Appears in these related concepts: Nuclear Fusion, Millikan's Oil Drop Experiment, and Electrolysis Stoichiometry
 electron
 Appears in these related concepts: Periods 1 through 3, Electrolytic Properties, and Microscopy
 energy
 Appears in these related concepts: Surface Tension, Energy Transportation, and Introduction to Work and Energy
 fraction
 Appears in these related concepts: SI Unit Prefixes, Separable Equations, and Fractions
 frequency
 Appears in these related concepts: Properties of Waves and Light, Characteristics of Sound, and Sound
 ground state
 Appears in these related concepts: The Third Law of Thermodynamics and Absolute Energy, Fluorescence and Phosphorescence, and Glow of Space Shuttles
 kelvin
 Appears in these related concepts: Celsius Scale, Speed of Sound, and Osmotic Pressure
 liquid
 Appears in these related concepts: Overview of Atomic Structure, Types of Synthetic Organic Polymers, and Three States of Matter
 liter
 Appears in these related concepts: Effect of a Common Ion on Solubility, Solution Stoichiometry, and Molar Solubility and Relative Solubility
 mole
 Appears in these related concepts: Avogadro's Number and the Mole, Molar Mass of Compounds, and Concept of Osmolality and Milliequivalent
 period
 Appears in these related concepts: Frequency of Sound Waves, Sine and Cosine as Functions, and Tangent as a Function
 substance
 Appears in these related concepts: DipoleDipole Force, Substances and Mixtures, and Complex Ion Equilibria and Solubility
 system
 Appears in these related concepts: Free Energy Changes for Nonstandard States, Definition of Management, and Comparison of Enthalpy to Internal Energy
 temperature
 Appears in these related concepts: Extractive Metallurgy, Temperature, and Microstates and Entropy
 thermodynamics
 Appears in these related concepts: Absolute Zero, Solutions and Heats of Hydration, and Work
Sources
Boundless vets and curates highquality, openly licensed content from around the Internet. This particular resource used the following sources: