The diagram below shows some subatomic particles. A diagram similar to a flowchart starts with a circle labeled Atom. From there, a line leads to a small circle labeled Electron and a larger one labeled Nucleus. From nucleus, two lines lead to a group of circles, labeled Neutrons and protons. From there, two lines lead to a circle surrounding two blue and one orange smaller circles. One of the blue circles is labeled with a question mark. What is the particle that is labeled with a question mark (?) in the diagram?