There are primarily three subatomic particles in an atom;
Protons
Electrons
Neutrons
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Protons are positively charged subatomic particles in an atom. These particles give an element it's identity, based on it's 'Atomic Number', or how many protons are contained in the atom.
Electrons are negatively charged subatomic particles in an atom. These particles give an element it's reactivity, and are a major part in finding where an element lies on the Periodic Table by valence electrons. We can discuss more of this in PM if you'd like to know more about this. Electrons are also a major part of finding an element's 'atomic mass', or how many neutrons it has.
Neutrons are subatomic particles in an atom that have a neutral or 'no' charge. These are inside of the nucleus of an atom, as opposed to protons and electrons, which are farther outward of the center of an atom.
To answer your question, and reading from the text above, 'Neutrons' is your answer.
I hope this helps!
