What to say, and how to say it are two questions that constantly occupy a teacher’s mind. Compare two approaches to the introduction of the idea of a mole (in chemistry). My daughter was studying this topic, and I recalled always having to go back to the definition, so I looked up the NCERT grade 11 textbook to see how this idea is introduced.
The image attached above is a page from a chemistry chapter for class 11 – NCERT textbook. I am usually a big fan of NCERT textbooks, but I think this part was a bit of a miss.
It is an introduction to the idea of a mole. It starts off fine. But, in the third and fourth paragraph, it gets unnecessarily complicated. Firstly, it says that mole is a base quantity for the amount of a substance. Then it says that the mole is an SI unit. It contains 6.022*10 (power 23) elementary entities. In the next few sentences, the author has lost me (entity, mass spectrometer etc.). Towards the end, just when I’m about to give up, the text gets to the single phrase of any use to the student (in this case, me): the molar mass in grams is numerically equal to the atomic/molecular formula mass in u.
Contrast this introduction to the idea of a mole in another chemistry book (by Rebecca W. Keller).
The masses of atoms are most conveniently measured in atomic mass units or amu. Each proton and each neutron has a mass of 1 amu. Hydrogen, with one proton and no neutron, is 1 amu, carbon, with six protons and six neutrons, has a mass of about 12 amu, and a gold atom, with 79 protons and 118 neutrons, has a mass of 197 amu. These values are only estimates…
Atoms are very small, so the mass of one atom is also very tiny. …Atoms are much too small to count one by one, so chemists count them in big groups, called moles. A mole is defined such that 1 mole of carbon atoms, each with a mass of 12 amu, weights 12 grams. In general:
A mole of atoms weighs the same (in grams) as its atomic mass (in amu).
Then, after giving several examples mapping moles to grams (example, a helium atom has a mass of 4 amu, so how many moles are there in 16 grams of helium?) she says:
A mole is just a name that represents a certain number of atoms, molecules, ions or even umbrellas. For example, we count 12 eggs by calling them a dozen. In the same way, chemists count by groups called moles.
A mole is 6.022 *10 to the power of 23.
It is important to note that one of mole of any atom is the same number of atoms, no matter how much the sample weighs. One mole of carbon atoms has the same number of atoms as 1 mole of gold atoms, even though the carbon weighs 12 gms and the gold weighs 197 gms.
For a pure element, if you know the weight of your sample in grams, you can find the number of modes in your sample by dividing by the atomic mass.
Moles = weight in grams/atomic mass in amu.
The point of the story is that when she is introducing an important idea in chemistry, she does not complicate the story by talking, straight off the bat, about SI units, Avogadro constant, mass spectrometer etc.
She uses as few new and technical terms as possible as she introduces the key ideas. She gives many examples till the idea of amu, mole, and grams, and their relationship to each other, are lodged in the student’s head. Then, she moves on to more complex ideas.
Note to self as teacher: Stay carefully minimal when introducing a key idea.
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