Use white marshmallows for protons and colored marshmallows for neutrons. Use toothpicks to connect them together and be mindful of the amount of materials used.

Your turn:

Alpha particles and Helium nuclei have the same atomic structure, they're called alpha particles during radioactive decay. They both include 2 neutrons and 2 protons, try making this yourself.

Now, construct a stable Helium-3 nucleus with 1 neutron and 2 protons. Helium-3 is just another isotope, the same element but different mass, of Helium-4 in the first step. Notice how the neutrons and protons add up to create the atomic mass. 

Helium-3 and Helium-4 were stable, but Helium-5, not so much. Make a He-5 nucleus using 3 neutrons and 2 protons. 

Next, let's switch it up and create a nucleus for Carbon-12 with 6 neutrons and 6 protons. This element is very stable because of it's neutron to proton ratio. 

Lastly, take the Carbon-12 nucleus and add it to the alpha particle made in step 1. New elements can be created by combining nuclei together. What new element is made? Find it on the periodic table. 

Find the full activity pdf with pictures here.