Why do I despise high school chemistry? Because it’s described in the antiquated terms of early 20th century science.
I’m talking about you, Avogadro constant.
I was tutoring my son this week when I came to a realization – we don’t need the Avogadro constant anymore.
It was useful before the existence of the atoms and molecules were established. But in 2007, we know about matter at the atomic and molecular scale, so we should stop replacing the concrete picture of subatomic particles, atoms and molecules with the (in)convenient ideas implied when we talk about moles of material.
The Avogadro constant is really just a bad approximation of one divided by the mass of the proton. It’s a bad approximation because instead of using the actual mass of the proton to calculate the constant, the accepted value is one twelfth the mass of a carbon 12 atom. The error comes about for several reasons. For one thing, there are twelve particles in the nucleus of carbon 12, but only half are protons, the other half are neutrons (which are heavier than protons, leading to a small over estimate). There are also six electrons floating around a normal (that is, neutral) carbon 12 atom, leading to another error from over estimating.
All of these things stick together to form one atom, which leads to yet another error. The binding energy that holds atoms together reduces the mass of carbon 12 by a lot more than the added mass of heavier neutrons and the extra electrons.
Put all these things together, and one divided by Avogadro’s number is nearly equal to the mass of a proton in grams. How annoying is that?
Why do I hate the Avogadro constant? Well kids, I have loads of reasons. But here’s three.
— If we use proton mass in chemistry calculations instead of the Avogadro constant, then you could lighten your load of constants that you need to know by one at least. Sure, I’ll have to memorize or look up the mass of the proton now, but I have to look that up on occasion anyway. I hate memorizing stuff, so this is a big deal to me. As a bonus, you can forget about the ideal gas constant (R) too. We only had to make that one up to atone for inventing the Avogadro constant in the first place.
— Instead of struggling to remember the abstraction of moles, we could just think of the actual constituents of molecules in balancing chemical equations, leading to a more clear understanding of what’s going on in chemistry. For example, Wikipedia says ‘A mole is much like “a dozen” in that both units can describe any set of elementary objects . . .‘ In other words, using the Avogadro constant in chemistry makes as much sense as going to Dunkin Donuts a and asking for a twelfth of a dozen donuts when you only want to buy one, or one and a twelfth dozen when you want 13 (which can also be written 1.0833333333 dozen donuts).
— The ideal gas law would make a lot more sense. What we’re really talking about in the ideal gas law is particles bouncing off of the walls of a container, so PV=nRT is really PV=NkT, where ‘N’ is the number of particles in the container, and ‘n’ is the number of 6.0221415×10^23 sized batches of particles in the container. ‘N’ is much more sensible than ‘n’, and takes a LOT less oxygen to describe.
My boss argued with the third point by essentially paraphrasing this Wikipedia entry about rationale behind moles –
Moles are useful in chemical calculations, because they enable the calculation of yields and other values when dealing with particles of different mass.
Number of particles is a more useful unit in chemistry than mass or weight, because reactions take place between atoms (for example, two hydrogen atoms and one oxygen atom make one molecule of water) that have very different weights (one oxygen atom weighs almost 16 times as much as a hydrogen atom). However, the raw numbers of atoms in a reaction are not convenient, because they are very large; for example, just one mL of water contains over 3×10^22 (or 30,000,000,000,000,000,000,000) molecules.
I don’t get it, why would chemists be upset by 3×10^22 (or 30,000,000,000,000,000,000,000), when a number approximately equal to 6.022^23 (or 602,200,000,000,000,000,000,000) is not a problem? Just round to three significant digits and use exponential notation, for crying out load.
In fact, using moles actually forces us to talk about the number of a number of particles. I’d rather just talk about the number of particles.
Here’s the bottom line: all you chemists using the Avogadro constant, hitch up your belts and move along from the science of the early 1900’s to the science of the late 1910’s and beyond by ditching that crazy constant.
Then, perhaps, students can start learning about chemistry as it happens for real.