It varies on the element of the atom.
The only subatomic particles that exist in an atom are protons, neutrons, and electrons.
In the atom these particles are the electrons.
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there are 80 protons in the nucleus of an atom of mercury
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There are estimated to be around 1080 particles in the universe.
The neutral atom of polonium has 84 protons and electrons.
Electrons are the particles that circle the nucleus of an atom.
The electrons, are the negatively charged particles that are part of an atom. They hang out in what is called an electron cloud around the nucleus of the atom. We might further say that those electrons exist in what we term electron shells, and within energy levels within those shells.
Nothing except for the force carrier particles that are responsible for all of the forces in the atom. These particles are called virtual particles and do not necessarily exist, but they are responsible for particle interactions. The space is simply that- space; it can be interpreted as a vacuum or simply said to be nothing.
Protons and neutrons compose the nucleus of an atom. Most of the mass of an atom exists in the nucleus. Other smaller sub atomic particles such as quarks exist, but are usually not addressed until advanced physics or chemistry classes.
At least two: 1 proton + 1 electron = Hydrogen atom