Re: Model of an atom

For fourth grade, the Bohr model of the atom should suffice.

Get some ping pong balls, two of one color, and two of another color.
Glue them together into a cube.  This is the nucleus.

Drill a hole through two small marbles, or if you have no drill, go to
the nearest cheap-jewelry store and buy some beads with holes through
them.  

Make a loop out of a wire coat hanger, and string a bead on it.  Do the
same with the other bead.  You now have two electrons, with the circular
wire depicting their orbitals.

Glue the two loops together so that they criss-cross one another, and 
suspend the ping-pong ball "nucleus" in the very center with some fishing
line or nylon thread.

The atom you've made is helium.  The one-colored balls are protons, the
other-colored balls are neutrons, and the beads outside are electrons.