All samples of an element are composed of atoms with the same1. atomic mass2. atomic number3. # of nucleons4. # of neutrons

The answer is 2. Atomic number

The correct option is 2. atomic number

Explanation:

Although, one of Dalton's atomic theory suggests that atoms of the same element are alike in all aspect and are different from atoms of other element. While the latter part of this particular theory is true, the discovery of isotopes made this part of Dalton's theory to be modified because isotopes of the same element have different mass numbers (and are thus not alike in all aspect) but however still have the same atomic number.

Hence, it can be said that atoms of the same element have the same atomic number. Atomic number is the number of protons present in the nucleus of an atom.