We reviewed the atom video (not everyone had seen it) and created a model of a water molecule out of poster board, construction paper, and string.
The water molecule is great because it's so simple: one oxygen atom and two hydrogen atoms.
Here's my script for the lesson:
- Try chopping an apple into smaller and smaller bits. If you break a piece of matter in half, and then break it in half again, how many breaks will you have to make before you can break it no further? In ancient Greece, Democritus asked this question. He reasoned that it had to end at some point in the smallest possible bit of matter. He named this basic matter particle an atom. It means "indivisible" in Greek.
- What does an atom look like? Nobody can see them directly, they are so small. We have to imagine them. Physicists are people (mostly grown-ups but some teenagers ands kids too) who think about this stuff a lot. In 1897 (around the time our house was built) an English physicist named Joseph John Thomson, who lived in Cambridge and taught physics to students there, was experimenting with electricity. He discovered something unexpected: that atoms have a nucleus made of even smaller particles called protons and neutrons, and one or more electrons orbiting it much like planets orbit around the sun. The orbits of electrons are called shells. Now, over a hundred years later, almost everyone imagines atoms this way.
- J. J. Thomson also suggested another thing: that not all atoms are the same. Different atoms have different numbers of protons, neutrons and electrons. Atoms always want to have a certain number of electrons on their outer shell: two on the first shell, eight on the second.
- When you put atoms together, some of their electrons get shared between the atoms. This binds the atoms together, forming a molecule.
- A water molecule is formed when one oxygen atom binds with two hydrogen atoms. They do this because the oxygen atom has six electrons on its outer shell. How many is it missing to be happy? Two. The hydrogen atom has only one electron. How many does it need? One. So, when two hydrogens and one oxygen find each other and bind, they form a molecule of water: H
The children like chopping things up and putting water into smaller and smaller droplets. Also, pointing to things and asking each other "is this made of cells?" and "what is this made of?" (they know the right answer is always atoms but sometimes they know some of the molecules too).
PS. A good source on the history of atoms here.