Base Units in SI Help (page 3)

By — McGraw-Hill Professional
Updated on Sep 4, 2011

The Mole

The mole, symbolized or abbreviated by the lowercase nonitalicized English letters mol, is the standard unit of material quantity. It is also known as Avogadro’s number and is a huge number, approximately 6.022169 × 10 23 . This is the number of atoms in precisely 0.012 kg of carbon-12, the most common isotope of elemental carbon with six protons and six neutrons in the nucleus.

The mole arises naturally in the physical world, especially in chemistry. It is one of those strange numbers for which nature seems to have reserved a special place. Otherwise, scientists surely would have chosen a round number such as 1,000, or maybe even 12 (one dozen).

A Note About Symbology

Up to this point we’ve been rigorous about mentioning that symbols and abbreviations consist of lowercase or uppercase nonitalicized letters or strings of letters. This is important because if this distinction is not made, especially relating to the use of italics, the symbols or abbreviations for physical units can be confused with the constants, variables, or coefficients that appear in equations. When a letter is italicized, it almost always represents a constant, a variable, or a coefficient. When it is nonitalicized, it often represents a physical unit. A good example is s, which represents second, versus s , which is often used to represent linear dimension or displacement.

From now on we won’t belabor this issue every time a unit symbol or abbreviation comes up. But don’t forget it. Like the business about significant figures, this seemingly trivial thing can matter a lot!

Practice problems of these concepts can be found at: Units And Constants Practice Test

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