When a body is orbiting around a central mass, it has a centre of mass. When two bodies are orbiting around a central point, the centre of mass is shifting slightly, so there are two centres of mass.
Before we go too far, we have to understand what a proton and a neutron are, An atom consists of a nucleus, in which a prooton, or a number of protons and neutrons are bound together. The smallest nucleus is that of Hydrogen, which has a single proton. The next element on the periodic table, or list of all the elements, is Helium, which like Hydrogen is a gas, but it is also relatively neutral, or inert in its properties, meaning it doesn't react much with other elements. This is because it has an equal number of protons, and neutrons in the nucleus, two of each.
Why, you will be asking, is there not an element having one proton and one neutron as a nucleus. We don't really know. It could exist, but we haven't found it yet, or more likely, there is just not enough there to keep the nucleus together. Again you ask why, because the single proton of Hydrogen stays. We immrditely assume there is more to the proton than meets the eye, but because it is so small, too small to see, we simply wonder, for the time being, what its structure is. One thing we assume, correctly as far as we know, is that all protons are the same, no matter which element they are in, and likewise all neutrons are the same as one another.
Then we come to the realisation that it is the number of protons in the nucleus which makes an element unique. Hydrogen has one, Helium has two. It just happens that it is the number of electrons which gives an element its properties, which are electrical properties which determine how an element reacts with other elements. It also just happens that when there are two protons, there are also two neutrons. The next element with an equal number of protons and neutrons is Neon, which is also an inert gas. ....Four sub atomic particles are bound together by an exchange which is happening between the two protons and two neutrons...
The Helium nucleus can survive as a particle without any electrons, but when it enters an environment where there are electrons, it pretty quickly picks them up, often by tearing them away from other atoms. This is why alpha particles, because this is what a Helium nucleus, with two protons and two neutrons is called, is considered "radioactive. It will burn skin, which is the result of having electrons removed. it will usually not pass through paper or clothing, because it has only to gain two electrons to become Helium once again, and then it will rise, because Helium is lighter than air. Being lighter means the same "weight" takes up more space.