The diagram shows two helium nuclei fusing to form a larger nucleus. Red circles represent protons and gray circles represent neutrons. How many protons are in the resulting nucleus? How many neutrons are in the resulting nucleus? What element is produced by this reaction?
Regarding the first question, “How many protons are in the resulting nucleus?,” when we look at this nucleus, we count the number of red circles and we see there are four. Doing a quick double check, we see that if we add together the number of protons in our reactants, the two helium nuclei, we also get four. This confirms our understanding that there are four protons in the resulting nucleus.
When we consider the number of neutrons in the resulting nucleus, now we count the number of gray circles and we see there are three. This agrees with our count of neutrons in the reactants, our two helium nuclei. So we see that, indeed, there are three neutrons in the resulting nucleus.
Finally, when we want to see what element is produced in this reaction, we’ll look at the number of protons in the product. That’s because it’s the number of protons represented here by the pound sign, which tells us what atomic element we’re working with. That’s how they’re defined.
Sometimes, we’ll see an abbreviation for an atomic element with the mass number of that element — the total number of protons plus neutrons in the atom — listed at the top left and the atomic number — the number of protons that the atom has — listed at the bottom left.
In our case, our product nucleus has an atomic number of four because there are four protons and a mass number of seven because that’s the sum of the number of protons and neutrons.
When we look up on the periodic table of elements, what element has four protons?, we see that that element is beryllium, abbreviated Be. That’s the atomic element that’s produced in this reaction.