What Is A Mole?

According to the NIST, a mole (mol) contains "exactly 6.02214076 × 10²³ ;elementary entities". But what exactly does this mean? An elementary entity can be any particle or specified group of particles, such as atoms, molecules or ions. You could even count a mole of bananas - if you had 6.02214076 × 10²³ bananas, you would have one mole of bananas. The number 6.02214076 × 10²³ is named the Avogadro constant, N_A. This number arises from the number of atoms in 12 grams of carbon-12. In other words, if you put individual atoms of carbon-12 (the isotope of carbon containing 6 protons and 6 neutrons in its nucleus) onto a weighing scale until it read 12 grams, you would have put 6.02214076 × 10²³ atoms onto the scale. This isotope was chosen because it is relatively easy to obtain pure.