Have the children name things that they see around them in the class and at home. Ask them to list the material each of these items is made of. Once you have the list, ask them to tell you what those materials are made of. For example, if a table is made of wood, what is wood made of? Then you tell them that the smallest particle that makes all matter around us is the atom. Everything -- air, wood, water, metals -- is made of atoms.
Now that the children understand that the atom is the basic brick that makes all matter around us, they will like to know about the shape of the atom. How does the atom look? Draw the diagram of an atom showing distinctively the position of the protons, electrons and neutrons within it. This is the time when you can explain to the children that the minuscule atom is further divided into three components that have very specific properties -- the proton, neutron and electron.
The proton needs to be explained to a sixth-grader in the simplest possible terms. Looking at the diagram you have drawn of an atom, ask the children to tell you where a proton is found. The center of the atom, which contains the proton, is known as the nucleus. What are the special properties of the proton? It has a positive electric charge and weighs less than a billionth of a gram.
Similarly, ask about the electron. When the children identify it on the diagram, start talking about its special properties and compare it to the proton. The electron has a negative electric charge, which is opposite to that of the proton. The electron is much smaller than the proton, weighing 1/2,000th of the weight of the proton. The position of the electron within the atom is also different from that of the proton -- the electron orbits the nucleus of the atom.
The next component of the atom visible in the diagram is the neutron. As the name indicates, this part is neutral. In other words, it has no electrical charge. The position of the neutron in the atom is the same as the proton. Its weight, too, is exactly that of the proton.