Hello! Your answer is protons.
There are three main parts of any atom. The proton, neutron, and electron. The proton and neutron form the center of the atom, or the nucleus, while the electrons float around them.
The protons determine the element. The number of protons is the atom's atomic number. For example, if there were 6 protons, the element would be the one with an atomic number of 6, or carbon. If there was just one, it would be hydrogen as that has an atomic number of 1.
When you change the number of neutrons, you get isotopes. This changes the atomic mass, as neutrons' main impact is mass. This also can mainly change stability. When you change the number of electrons, you change the charge of an atom. Electrons' main purpose is to bond + charge the atom. Electrons don't weigh much, they barely weigh anything. They instead give a negative charge to an atom (while the proton gives a positive)
Hope this helps!
