Can the internet explain the difference between a molecule and a compound?

Every year I teach my students the difference between the words “molecule” and “compound”.  The difference is that these two words refer to two different concepts on two different size scales.  A molecules is a collection of 2 or more atoms held together by a strong force in a set structure.  Water (H2O), the oxygen in air (O2), the nitrogen in air (N2) and table sugar (C12H22O11) are common substances we encounter that are composed, at the atomic level, or molecules.

We will later will call this force a covalent bond, but that’s later on in the course.  A compound, however, is a pure substance that has a set ratio of two or more elements.  Water (H2O) is a compound that, at the atomic level, is composed of atoms.  O2, is composed of molecules but is not a compound.  It only contains atoms of one element, oxygen.   There is another class of compounds, the ionic compounds, that are compounds that do not form molecules at the atomic level.  Sodium chloride (NaCl, table salt) is composed of rows of ions (or a lattice), the ion pairs are not covalently bonded and will come apart (dissociate) when placed in water.  In other words, ionic compounds are compounds that are NOT composed of molecules.

Every year we have to spend some time on this because it is a persistent misconception that molecules and compounds are the same things.  It turns out that this is a much more persistent misconception than I thought.  If you Google “definition of compound and molecule”, you discover many sites that don’t correctly explain the difference between molecule and compound.  This site and this site and this site are just plain wrong.  They all have some variation on the INCORRECT statement that “all compounds are molecules” and/or “but not all molecules are compounds”.  This site (a .edu address!!) makes it sound like ionic compounds are formed from molecules.

In other words, not only should you not trust the internet, but this is one of those misconceptions that is hard to overcome, even for people who regard themselves as “experts”.  I will fully admit that I didn’t have this distinction clear until I was a chemistry teacher myself (and I have a Ph.D. …).


In celebration of the hard slog

It seems that everyone is focused today on Big Ideas and The Next Big Thing.  Maybe it’s because we live in a world of constant technological change that heightens our awareness of it.  Maybe it’s because Twitter and TED talks have put Big Ideas in nice bite sized packages.  It seems that everyone wants to shift the paradigm, change the optics, focus on the “Big Picture”.  Everyone wants to have a big idea that changes the world.

That’s fine. The big problems facing humanity require big ideas to solve.  However, lost in all of this is a recognition and a celebration of the necessity of the hard slog.  I come (and do most of us) from a long line of farmers, a group of people who understand constant, hard work better than almost anyone. I can’t keep the grass out of a 50 square feet of vegetable garden, let alone manage hundreds of acres of farm. But it is my career as an educator that has made me appreciate the importance of sustained hard work, the importance of incremental change and not just major changes and shifts.

Think about it this way. Let’s say you have one bad math teacher in high school for one year.  Over the course of a school year, you are now (50 minutes x 180 class periods/60 minutes per hour =) 150 hours behind someone that had a good math teacher. That is 150 hours of experience that a teacher later has to help you make up, because if they don’t you will fail his or her class too.  As a university professor I often encounter students who know barely any chemistry upon entering my class.  I also get students who are well prepared and can learn an enormous amount from me because their middle school and high school science and math teachers worked hard, every day over a period of years to teach and prepare them for the next level.

There is a great analogy for this in geology.  Mountains are built up because gigantic plates of the earth’s crust run into each other and force land upward.  Over millions of years, however, these same mountains are worn down by wind and rain into mere hills, then plains.  Tiny changes, unobservable as they happen but sustained over long periods of time, are just as important as tectonic forces.

The work of education, of both teaching and learning, is hard work. But it is that hard work that pays off for my students in the end.  It’s not because someone had the next Big Idea.  It’s because people concentrated and motivated themselves to work hard, day in and day out.  As a new school year begins we need to remember that both kinds of change, dramatic shifts and small but persistent changes, are important and need to be recognized.