The main 3 subatomic particles that makes up an atom are protons, neutrons, and electrons.
Protons and neutrons exists inside the nucleus of the atom, while electrons moves freely outside the nucleus.
Protons are positive in charge, each element has their own specific number of protons, it's just like their ID number for the element, they cannot be changed.
Neutrons are neutral, just like as it name says. Usually they have the same number as the protons in most elements, but not all cases. Sometimes an element may even have a few different forms, called Isotopes specifically, where they have different number of protons for each isotope.
And electrons, are negative in charge. They're a lot more lighter comparing to neutrons and protons, therefore sometimes we neglect their mass in calculating maths. In an atom, the number of electrons must be the same as the number of protons, or else, the atom would no longer be an atom, but an ion instead.
