Here are two short excerpts from my first book Teach Now! Science, The Joy of Teaching Science the 2014 Routledge series edited by Geoff Barton.
Across the entirety of a school life, students will develop an ever more sophisticated idea about the structure of matter and how materials are made up of particles – either as atoms, molecules or ions. It is very important to use these terms correctly and to challenge students if they mix them up at a point when they should know the difference. This is where coherence kicks in. You can’t teach about chemical reactions or vapourisation until students have a reasonable concept of materials being made up of atoms or molecules.
If we’re introducing particles, we might look at sugar or salt: two white granulated substances. It’s possible to imagine dividing them up into ever finer pieces until we start to imagine ‘building blocks’ that can’t be subdivided further. The aim is to develop a mental model for the idea that all substances are macroscopic manifestations of billions of relatively simple particles joined together.
At the beginning of this particular learning path, there is no point getting bogged down in distinguishing between salt as an ionically bonded giant lattice structure and sugar as a molecular compound. This is too much detail. Similarly, it is sensible to stop at ‘atoms’ without needing to delve further to discuss electrons, hadrons and quarks.
Once students can talk about particles, it is easier to discuss a wide range of phenomena:
- The difference between ice, water and steam despite being made of identical molecules.
- Digestion as the breaking down of large molecules into smaller ones that can fit through holes in the tissues that line the digestive system organs.
- Evaporation as process by which the most energetic molecules continually escape the surface of a liquid…until it has all vapourised.
- The thermal decomposition of green copper carbonate to form black copper oxide with the release of carbon dioxide.
- Diffusion of smells across a room as the random movement of gas particles interacting with air molecules via series of collisions.
- The idea that a person standing on a table on a macro scale is actually being held up by attractive forces between molecules of wood, balancing the force of gravity.
Without a strong model for particles, these phenomena at their everyday macro scale are virtually impossible to comprehend. “The puddle just disappears!” “The green stuff turns into black stuff!” “Ice reacts with heat to make water”… and so on.
It is important to pitch the learning at the right level so that you are not locking in incorrect ideas. I find that even very sophisticated students can lapse into using molecule, ion and atom almost interchangeably as if they don’t really matter, long after it really does! This then inhibits their understanding: Hydrogen gas reacts with oxygen as it does because it is a molecule H2. In it’s ionic form, H+, it behaves differently.
So your job is to map this out. At each new level of depth, you need to give students the time to reconfigure their mental models. It helps to refer to diagrams and visual models to develop a deeper understanding of atoms and molecules. As a teacher, a good understanding of the kinetic theory of particles is really important as it is something you return to over and over again.
However, take care with diagrams. An example often cited by Dylan Wiliam is of a teacher who showed her class a diagram of water molecules: lots of H=0=H arranged on a page. Students could reproduce this diagram on request to show what water looked like. Did this show they understood the molecular nature of water? No. Why? Because, when asked “where is the water?” several students pointed to the space in between the molecules; they’d failed to grasp that the molecules were the water.
Scale as a concept is rarely taught as a discrete topic but it permeates all topics to some degree. As part of forming mental models that, in turn, help students to explain key phenomena, working out the scale is important. Here, this isn’t about precision; it is about a rough order of magnitude. Often relative scale is expressed through analogies with macro-scale objects but we also need to try to see scale directly if at all possible.
In biology, scale is nicely dealt with by using microscopes. A typical blood cell, cheek cell or onion cell can be seen with a low power school microscope. Through microscope work, students start to realise that there are other worlds beneath the macro world they operate in.
Another example of this is the study of grass as an ecosystem. If you ask students to lie down on grass and peer into a localised, it takes on a totally different scale. All kinds of insects and plant varieties become apparent. You can also extract a lot of learning from catching insects in a ‘pooter’ trap, examining them with a simple magnifying glass. A concept of scale develops simply from changing the students’ perspective, asking them to look up close at things they see every day.
Getting the sense of the size of an atom is interesting but a lot more difficult. There are lots of websites, photographs and pieces of software that provide models of this. But how can you do it in a classroom? Try this:
Firstly, ask students to look at the lines on a ruler, mentally dividing each millimetre into 10 smaller divisions. They can just see this. They then have to visualise mini-cubes that are 0.1mm x 0.1mm x 0.1mm in size. Now, imagine a line of cubes along a 1 metre rule. There will be 10,000 of them.
If we scale up the model further to a 100 metre running track there will be 1 million cubes in a line. Dwell on this for a while to get students to visualise this line. Finally, imagine that the whole line of 1 million cubes is shrunk down to fit along the edge of just one of the 0.1 mm cubes. We now have 1 million cubes sitting along a 0.1mm line. That’s about the size of an atom. It is head-spinning and, for most purposes, you can gloss over this. However, it pays to invest in this kind of model building repeatedly,
The size of a nucleus relative to an atom is another classic scale model. A common example is to say it is like a pea at the centre of a football pitch, with the electrons whizzing around the stands. The key learning here is that most of an atom is empty space! For many students all this is very abstract and they will barely grasp it. However, there will be students who can deal with this complexity and will want to grapple with it. Try linking this atomic model to the cubes-on-the-ruler model..the stadium needs to shrink down to fit into one of the shrunken mini-cubes!
Another important idea is the number of atoms in a mole of a substance. This helps get a sense of atomic size but also puts chemical and biological processes into perspective. The number of atoms in 12 grams of Carbon-12 is 6.02 and 1023. Avogadro’s number. How big is this? One way to visualise it is to compare with grains of sand on a beach: there are more atoms in a grain of sand, than grains of sand on a beach. Or, the one I like is to reference it to time. The universe is about 10 billion years old and, in each year, there are over 30 million seconds. So, if we counted the atoms in a pin head, one atom each second, it would take longer than the age of the universe.
A similar scaling process is useful to capture the size of our solar system or the distance to the nearest star after our Sun. Moving up from Earth-Moon, to Earth-Sun, to solar system, galaxy and distance to the next nearest galaxy are all big questions that require a mental model of sorts. Describing the large scale of our universe is important in understanding the limitations on space travel and in developing a proper understanding of just how remote Earth is.
In science generally, knowing the size of any quantity is useful. From life experience, we know the size of a metre and the length of a minute relative to other lengths and times. But most other units need to be given a context. Whenever you introduce a new quantity with new units, it is useful to put it in perspective. Teslas and Coulombs are huge units for magnetic field strength and charge respectively. A 100 amp current would be enormous (domestic fuses are 13 Amps); 100 m/s is 10 times faster than Usain Bolt; 100 Pascals is a very low pressure – atmospheric pressure is 100 kPa, the equivalent of carrying several hundred kilograms on our heads all day!
Scale in Perspective.
Although I’ve just outlined some ideas for exploring the true scale of numbers of particles and their size, we mustn’t lose sight of the fact that we all experience science on a macro real-life scale. Models are only approximations and we are asking students to make a very significant leap in relating our simplified diagrams to the macro phenomena they observe with their own eyes.
The trick is to continually make references back and forth from the models to the macro. The stylised, idealised plant ‘form and function’ diagram showing all the cellular structures in the roots, stem and leaves is an essential piece of learning. The photographs of microscope slides showing specimens of root hairs or stomata guard cells are fascinating. But you also need to set them alongside some real plants so that students can make the connections in terms of their own sense of scale.