The mole - unit and concept

Featured paper

Fang, S.C., Hart, C. and Clarke, D. (2014) 'Unpacking the meaning of the mole concept for secondary school teachers and students', Journal of Chemical Education, 91, pp. 351-356.

This paper reports the findings of an in-depth content analysis relating to the concept of the mole. This content analysis was used by the authors to form of a concept map. 

Unlike other SI units such as the gram and the second, the mole is often described as the "mole concept". The concept map provides a visualisation of the interconnected sub-concepts that are needed to understand the wider mole concept. The authors suggest that the concept map could be a useful tool for teachers in thinking about how they could meaningfully teach the mole concept to students.

Please note that the authors used the  IUPAC definition of a mole that was valid at the time of publication  rather than the most recent version.

A mole is the amount of substance which contains as many elementary entities as there are in carbon atoms in 0.012kg of carbon-12.  When the mole is used, the elementary entities must be specified and may be atoms, molecules, ions, electrons, other particle or specified groups of such particles.

The authors created a concept map (illustrated below) to show how specific sub-concepts connect to enable an overall understanding of the mole concept. 

The authors describe the idea of the amount of substance (in moles) as being the bridge that links the number of elementary entities to the mass of a substance.

The concept map includes some sub-concepts that are linked to understanding of atoms, molecules and relative atomic and molecular mass as well as sub-concepts linked specifically to the mole concept. Identification of the importance of the atomic-molecular concept in understanding the mole concept was a key outcome of the content analysis. 

The atomic-molecular sub-concepts link to the mole concept in two different ways.

Link 1 - connecting the sub-concept of atoms and molecules to the number aspect of the mole concept

This idea connects the concept of a single atom or molecule (or other elementary entity) with the idea of thinking about a ‘standard pack’ of atoms or molecules called the mole.
This idea of aggregating atoms or molecules into a “standard pack” does not explain to students why the number 6.02x10^23 was chosen as the number in this “standard pack.” This number is not mentioned in the original definition of the mole.
The authors suggest that students are taught that the number of atoms in 12g of carbon-12 was experimentally determined to be 6.02x10^23.

Link 2 - connecting the sub-concept of relative atomic or molecular mass to the mass aspect of the mole concept

By definition, one mole of any substance always has the same number of elementary entities as 12g of carbon-12. If the number of elementary entities of different elements are the same, then the ratio of their masses will be the same as the ratio of their atomic or molecular masses.
Due to the choice of 12g of carbon-12 being the measure of one mole, the relative atomic mass of any other element in g will also contain one mole of atoms.

The authors alert teachers that this understanding requires students to use proportional reasoning. 

Consequences for teaching

The authors recommend that in order to meaningfully teacher students about the mole they should guide students to make the connection between the atomic-molecular concept to both the number and mass aspects of the mole concept. 

BEST Diagnostic Question

Relative atomic mass

Every element has a relative atomic mass.
You can find the relative atomic mass of an element in the Periodic Table.
Which answer best states the relative atomic mass of helium?

A 4g
B 4mg
C 4
D 4x10^-9g

The correct answer is A. Relative atomic mass does not have a unit.
A student who chooses option A may hold the misconception that relative atomic mass is the same as the molar mass, which is measured in g.
Selection of option B or D may suggest that a student thinks that relative atomic mass is the mass of an actual atom (which is smaller than a gram).

Reflective questions

How do you first introduce the mole? 

What connections do you make with earlier understanding?

To what extent does the current definition make the mole concept easier or more difficult to understand?

The current IUPAC definition of a mole is:
The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.02214075x1023 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in mol^-1, and is called the Avogadro number.

Useful links

BEST Topic 6 Key Concept 1: Amount of substance

Diagnostic questions to check for student misconceptions about the atomic model as part of a five-part progression (and including response activities)

University of York Science Education Group