The SI unit for amount is the mole, abbreviated mol. It is defined as the amount of substance that contains as many entities (atoms, molecules, or other particles) as there are atoms in exactly 0.012 kg of pure carbon-12 atoms. Many experiments have refined the number, and the currently accepted value is
This number, often rounded off to 6.022 x 1023, is called Avogadro’s number in honor of Amedeo Avogadro (1776–1856)
He atoms, so one mole of helium consists of 6.022 x 1023 He atoms. Hydrogen commonly exists as diatomic (two atom) molecules, so one mole of hydrogen is 6.022 x 1023 H2 molecules and 2(6.022 x 1023) H atoms.
Worked Example
How many atoms are contained in 2.451 mol of iron?
Plan
One mole of atoms of an element contains Avogadro’s number of atoms, or 6.022 x 1023 atoms.
This lets us generate the two unit factors :
Solution
We expected the number of atoms in more than two moles of atoms to be a very large number. Written in nonscientific notation, the answer to this example is: 1,476,000,000,000,000,000,000,000.
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