The easiest way to explain atoms, molecules and matter to kids, is with visual displays or games. Just reading a book or seeing things on a chalkboard, will not impress the average student. Nor will it inspire them to read further, let alone consider a science career. Understanding these broken down substances of matter is key to all the sciences, not just physics. Here are a few tips of visual displays or games, to help kids in the classroom understand and hopefully get excited about science.
Draw a huge pyramid on the chalkboard. Write Atomic Particles on the bottom line, and write My Body at the top tip of the pyramid. This is a very basic visual sense of what a human body is constructed of. If this is done mid-way or at the end of a science class, ask for a show of hands of the exact sequence of matter, from Atomic Particles upward (electrons and protons, atoms, molecules, and so on).
If this is a first-day science class, explain how you will be filling in that pyramid in that class. You might also ask for a show of hands about what fills in the pyramid, to gauge what the students already know, or what concepts they struggle with.
Use magnetic buckyballs or even beads (in three colors, if possible) to represent elementary particles. Place a small pile of counted-out buckyballs/beads in two colors on a desk in the center of the classroom. Then put a specific number of folded 3X5 cards one of the third color buckyballs/beads on certain outer desks of the classroom. Let them guess what is represented, or tell them that this represents a much-magnified Atom.
Explain that the central buckyballs/beads are the atomic nucleus, made up of positively charged protons and electrically neutral neutrons. And they are bound together similar to the magnetic buckyballs, by electromagnetic force. Then explain that the tented cards; and/or single buckyballs/beads represent the surrounding cloud of negatively charged electrons in an atom.
This visual display will give kids an idea what atoms look like, greatly magnified. Then explain that an atom is "classified" according to the number of the proton and neutrons in the nucleus: the proton number defines the "chemical element", and the neutrons determines the "isotope" of the element.
After explaining these elementary particles, write the number of the buckyball/bead protons and neutrons on the board, and point to the Periodic Chart, hopefully on the wall. Let the kids figure out what the chemical element and isotope is represented by the Classroom Atom.
Write a list of topics on the board, and ask the kids to chose one and write a one page report on it, due in a few days. Let the kids chose their own topic. Don't mandate that everyone has to chose a different topic, or forbid multiple kids from writing on the same topic. This will give the teacher an idea what atomic elements or topics the kids are most interested in. And it will also give the teacher an idea that the topics not chosen need more rigorous instruction, or a different approach.
Depending on the age of the students, here are some suggestions for topics, which can range from explaining a proton, neutron or electron; to the fascinating history of how early scientists theoretically started to understand atomic structure, long before super-microscopes and super-colliders could prove the existence of atomic particles. Also suggest a topic on the CERN super-collider in Europe, and/or a topic about the Big Bang. You may want to later let certain papers be read aloud in class, and invite questions to help clarify the classes' understanding of the topic.
Explain how certain atoms can remain bound to each other to form molecules, through covalent chemicals or other types of bonds. Again, a Classroom Molecule can be set up, similar to the Classroom Atom. And, after explaining the process of how atoms form molecules, let the kids figure out what kind of molecule the Classroom Molecule represents. Homework could be assigned, for the kids to list as many substances as possible that the represented what the Classroom Molecule is part of.
Again, a visual display can be fun and help the kids understand. Set up three examples of matter in the classroom, for the kids to study and learn. It can be simple yet fun. A jar of rocks can represent a Solid State of Matter, and a jar of water can demonstrate the Liquid State of Matter. End with an empty jar to represent Gaseous State of Matter. Let the kids guess what each jar represents, before the classroom lesson.
Especially see how many of them guess correctly about the empty jar. You may want to start with a sealed jar, then remove the lid and say that the substance (gases) is still represented in that now open jar.
Science is fascinating and fun. And teachers can enthuse students through simple, inexpensive visual displays like these.