Definition: The mole is defined as the atomic or molecular weight of a substance expressed in grams. It is a metric unit for amount of substance and has the abbreviation mol.
Atomic weights were originally determined by weighting equal volumes of gases which according to Avogadro's Hypothesis contain the same number of particles. Since they contain the same number of particles, the mass of the equal volumes also represented the mass of the particles that comprised the gas. Guy Lussac's work on the combining volumes of gases helped chemists determine the number of atoms in each molecule of the gases, and therefore allowed chemists to determine the relative weights of the atoms that made up the gases.
In order to make this information useful chemists then picked a standard of atomic weight that they compared all values to.
- Originally the standard was the oxygen molecule since so many of the known elements combined with oxygen. It's value was set at 32 g.
- This value was chosen because it made the value of a hydrogen molecule 2 grams and the value of a hydrogen atom 1 gram.
- Today the standard is the isotope carbon - 12 .
- Atomic weights today are determined using a mass spectrometer. This instrument measures the atomic weight very precisely and can be used for all elements, whether in a gas form or not.
- Atomic weights are listed for each element in the periodic table.
A twist in logic allows chemists to apply their knowledge of atomic weights to develop a new measure for the amount of substance in a sample. As long as the relative weights are the same a sample of elements or compounds contains the same number of particles. Since Atomic weights were measured using equal number of particles, chemists simply express the relative weights (atomic weight) in grams in order to always have the same number of particles.
One mole of a substance then is the atomic weight of the element expressed in grams.
The number of particles in this amount is termed Avogadro's Number and has been estimated at 6.023* 10 23 particles. Whenever we have Avogadro's number of particles in a sample we have one mole. This number is almost unimaginable. To give you a sense of it's size, imagine that each particle in a mole was a piece of paper. If we were to stack this paper one sheet on top of another, a mole of paper would stretch from the surface of the earth to the planet Pluto.