Which subatomic particle determines the identity of a chemical element?

The number of protons in the nucleus, called the atomic number, is what defines an element. One proton makes hydrogen, six makes carbon, seventy-nine makes gold, and no amount of adding or removing other particles changes that. The periodic table is simply the elements laid out in order of proton count, which is why the sequence has no gaps. Change the number of neutrons and you still have the same element, just a different isotope, which may weigh more and may be radioactive but behaves the same chemically. Carbon-12 and carbon-14 both make the same bonds. Change the number of electrons and you get an ion with a net charge, which alters reactivity but not identity. The only way to turn one element into another is to change the nucleus itself, through radioactive decay, fusion or bombardment in a reactor or accelerator, which is why the alchemists' chemical approach was doomed.

Elements beyond uranium are made artificially by smashing lighter nuclei together to build up the proton count.