The last four units in the SI system, the mole, kelvin, ampere, and kilogram, all form a knotted tangle of definitions. Until 2019, they were all defined almost independently. The ampere had its own definition and so did the kelvin. The mole and kilogram were then related. However, all these definitions were unsatisfactory, especially the kilogram which was still defined as the mass of an artifact. Then, in 2019, the kelvin, ampere, and kilogram were simultaneously redefined by relating temperature to energy and energy to mass. Once the kilogram was redefined, the mole could also be redefined. What follows is an explanation of the mole’s part in this grand redefinition.
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To understand all this, we first need a basic understanding of the mole. This unit measures the amount of substance. This “substance” can be any discrete unit: atoms, molecules, ions, or even dollar bills. The mole itself is a set number of these units, about 6.022 x 10^23 (called Avogadro’s Number after the man who pioneered the mole in the 1800’s). That means that one mole of, say, copper contains about 602 200 000 000 000 000 000 000 atoms. This number is so large that it is hard to understand. If you had one mole of dollars, you could spend one trillion dollars every second from the moment you were born until you died at age 90, and you would only have spent less than 0.5% of your wealth.
So where do we use such a number? This number is very useful in chemistry, where comparing masses of elements is useless when referencing chemical formulas. For example, water has formula H2O. This means there are two hydrogen atoms for every oxygen atom. However, we cannot then say that there is one gram of oxygen for two grams of hydrogen because one oxygen atom masses more than a hydrogen atom (Fig 2). Thus, we use the mole to convert between an easily measurable mass and an immeasurable number of particles.
It is this mass-number conversion that has defined the mole for much of its history. Up until 2019, it was defined as the number of atoms in 12 grams of carbon-12 (a type of carbon). Obviously, this left much to be desired because the kilogram was defined in terms of a variable artifact. This variation in the kilogram made the measurement of Avogadro’s Number impossible. Only with the redefinition of the kilogram could scientists produce an accurate count of Avogadro’s Number, thus affixing it as a natural constant and defining the mole. Now the mole is defined as:
“The mole, symbol mol, is the SI unit of amount of substance. One mole contains exactly 6.022 140 76 x 10^23 elementary entities. This number is the fixed numerical value of the Avogadro constant, NA, when expressed in the unit mol-1 and is called the Avogadro number. The amount of substance, symbol n, of a system is a measure of the number of specified elementary entities. An elementary entity may be an atom, a molecule, an ion, an electron, any other particle or specified group of particles.”
Which, admittedly, is not much different from the old definition. However, it has an added layer of accuracy and stability that allows the meteorologists to sleep at night. That about wraps up the mole, and next week I will talk about the kelvin, the unit of temperature. See you then.
PS. In case you were wondering, no. The mole is not named after the animal. It is derived from the word “molecule” and was popularized by a German scientist.
I had no idea the mole was redefined last year! I guess I probably would have heard more about it if I was taking a chemistry class; the last science class I took was in 2018. Anyways, this was a really interesting article! When I was in tenth grade, we had an extra credit assignment where we had to make a stuffed animal mole to celebrate Mole Day (10/23) so every time I hear or see something about the unit of measurement, I just think about that memory.
I always thought that the mole was such an interesting unit of measurement. When I learned about it back in High School, it was always so abstract. Our chemistry teacher taught it to us as if it were something like a dozen. For example, a dozen is always 12 of something; a dozen pencils is 12 pencils, a dozen phones is 12 phones. This blog puts the concept into much clearer terms and makes the whole concept make more sense. I think part of my confusion was the incomprehensible size of a molecule and a mole. The example I was given was that if you filled a grapefruit with nitrogen atoms, you could fill the earth with an equal number of basketballs. Overall, a very insightful blog that I genuinely enjoyed.
The mol has always been a peculiar unit to me. I began to understand its use in my chemistry courses, but I forget why 6.022 instead of any other number. I am sure there is a good reason, but it is not something I have committed to memory. Also, I had no idea as to how loosely defined it was. I assumed that there was a concrete standard for it prior to 2019, but I never looked into it. I also wonder if anyone uses it outside of chemistry. Its such a specific and large number that I doubt it has any other practical uses. It is still very interesting to learn about all the SI units!
I learned about the mole when I took Chemistry in my junior year of high school but I think this blog taught me more than that whole year of class did lol. That class was such a struggle which shows why I really have no recollection of this measurement, but wow, I would love to have one mole of dollars. Is it even possible to measure things other than elements with it?