Question

What must two atoms have the same if they represent the same element?

Answer 1

An atom is consist of a nucleus in which protons and neutrons are present where protons are positively charged species while neutrons are of neutral nature and same number electrons as that of protons are revolving around the nucleus and the overall charge on the nucleus in neutral.

Complete answer:
Atomic number of any substance is always a whole number as it is obtained by counting electron, proton and neutron in an atom. Atomic number is defined as the number of protons present in the nucleus and it is generally represented by the symbol Z. Atoms of particular elements have the same atomic number as they consist of the same number of protons.
Atoms of different elements have different atomic numbers. Each element has its particular atomic number from which it gets recognized no two elements have the same atomic number like oxygen have atomic number 8 while hydrogen have one.
Hence from the above discussion we can conclude that two atoms must have the same atomic number if they represent the same element which means they consist of an equal number of protons. Mass number of the same element can be different but the atomic number always remains the same for the same elements.

Note:
Modern periodic table is arranged in such a manner that molecules are arranged according to their increasing atomic number where hydrogen is placed at the first position of periodic table which suggest that atomic number of hydrogen is one.