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Avogadro's Number

Defines concept of the mole as a counting unit

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Avogadro's Number

It is often convenient to have a unit for large amounts, such as a mole

Credit: User:OSX/Wikimedia Commons
Source: http://commons.wikimedia.org/wiki/File:1983-1988_Toyota_Hilux_4-door_utility_01.jpg
License: CC BY-NC 3.0

Is there an easier way to load this truck?

When the weather is nice, many people begin to work on their yards and homes. For many projects, sand is needed as a foundation for a walk or to add to other materials. You could order up twenty million grains of sand and have people really stare at you. You could order by the pound, but that takes a lot of time weighing out. The best bet is to order by the yard, meaning a cubic yard. The loader can easily scoop up what you need and put it directly in your truck.

Avogadro’s Number

It certainly is easy to count bananas or to count elephants (as long as you stay out of their way). However, you would be counting grains of sugar from your sugar canister for a long, long time. Atoms and molecules are extremely small – far, far smaller than grains of sugar. Counting atoms or molecules is not only unwise, it is absolutely impossible. One drop of water contains about 1022 molecules of water. If you counted 10 molecules every second for 50 years without stopping you would have counted only 1.6 × 1010 molecules. Put another way, at that counting rate, it would take you over 30 trillion years to count the water molecules in one tiny drop.

Chemists needed a name that can stand for a very large number of items. Amedeo Avogadro (1776 - 1856), an Italian scientist, provided just such a number. He is responsible for the counting unit of measure called the mole. A mole (mol) is the amount of a substance that contains 6.02 × 1023 representative particles of that substance. The mole is the SI unit for amount of a substance.  Just like the dozen and the gross, it is a name that stands for a number. There are therefore 6.02 × 1023 water molecules in a mole of water molecules. There also would be 6.02 × 1023 bananas in a mole of bananas, if such a huge number of bananas ever existed.

Portrait of Amedeo Avogadro

Credit: C. Sentier
Source: http://commons.wikimedia.org/wiki/File:Amedeo_Avogadro2.jpg
License: CC BY-NC 3.0

Italian scientist Amedeo Avogadro, whose work led to the concept of the mole as a counting unit in chemistry. [Figure2]

The number 6.02 × 1023 is called Avogadro’s number, the number of representative particles in a mole. It is an experimentally determined number. A representative particle is the smallest unit in which a substance naturally exists. For the majority of elements, the representative particle is the atom. Iron, carbon, and helium consist of iron atoms, carbon atoms, and helium atoms, respectively. Seven elements exist in nature as diatomic molecules and they are H2, N2, O2, F2, Cl2, Br2, and I2. The representative particle for these elements is the molecule. Likewise, all molecular compounds such as H2O and CO2 exist as molecules and so the molecule is their representative particle.  For ionic compounds such as NaCl and Ca(NO3)2, the representative particle is the formula unit. A mole of any substance contains Avogadro’s number (6.02 × 1023) of representative particles.

The animal mole is very different from the unit mole

Credit: Left: Michael David Hill, 2005 (Mikiwikipikidikipedia); Right: chrisbb@prodigy.net
Source: Left: http://commons.wikimedia.org/wiki/File:Close-up_of_mole.jpg; Right: http://www.flickr.com/photos/chrisbrenschmidt/436990097/
License: CC BY-NC 3.0

The animal mole is very different than the counting unit of the mole. Chemists nonetheless have adopted the mole as their unofficial mascot. National Mole Day is a celebration of chemistry that occurs on October 23rd (10/23) of each year. [Figure3]

Summary

  • A mole of any substance contains Avogadro’s number (6.02 × 1023) of representative particles.

Practice

Questions

Use the link below to answer the following questions:

http://www.scientificamerican.com/article.cfm?id=how-was-avogadros-number

  1. What was Avogadro’s hypothesis?
  2. Who first calculated this number?
  3. Who coined the term “Avogadro’s number”?
  4. What contribution did Robert Millikan make to the determination for the value for the number?

Review

Questions

  1. What is the SI unit for amount of a substance?
  2. What is the representative particle for an element?
  3. The formula unit is the representative particle for what?

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    Image Attributions

    1. [1]^ Credit: User:OSX/Wikimedia Commons; Source: http://commons.wikimedia.org/wiki/File:1983-1988_Toyota_Hilux_4-door_utility_01.jpg; License: CC BY-NC 3.0
    2. [2]^ Credit: C. Sentier; Source: http://commons.wikimedia.org/wiki/File:Amedeo_Avogadro2.jpg; License: CC BY-NC 3.0
    3. [3]^ Credit: Left: Michael David Hill, 2005 (Mikiwikipikidikipedia); Right: chrisbb@prodigy.net; Source: Left: http://commons.wikimedia.org/wiki/File:Close-up_of_mole.jpg; Right: http://www.flickr.com/photos/chrisbrenschmidt/436990097/; License: CC BY-NC 3.0

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