It’s been a while since I was taught Chemistry, but I was reading something the other day and I couldn’t quite get my head around the concept.

So the periodic table shows elements with their atomic numbers (protons) and atomic weight (protons + neutrons). Isotopes have different numbers of neutrons, like carbon-12 and carbon-13 which I believe would have 6 and 7 neurons respectively.

So why is carbon-12 the ‘default’ option, shown on the periodic table? Is it to do with which version of carbon is the most common? I don’t understand how we decide what makes up the pure version of an element/ it’s atomic weight as shown on the periodic table