[alirdz]

Hi, I'm wondering which of these makes an element unique from other elements.
- atomic number
- mass number
- atomic weight

I know that the atomic number (number of protons) makes it unique, but I'm confused as to if any of the others are unique to each element too. I remember that Dmitri Mendeleev arranged his periodic table by atomic weight, so does that mean that atomic weight is also unique to each element? I'm not sure about mass number.

Thank you all so much for your help in advance 

[kd.gns]

Mendeleev accounted for properties too when listing by atomic weight, see Te vs I. The mass number is not exactly unique when you consider isotopes, a different isotope of an element could exhibit the same properties (if its stable) except for the mass, and is still considered the same element.

[eggles]

the number of protons in the nucleus eg atomic number