What must all atoms of the same element have in common?

All atoms of the same element must have a common atomic number. The atomic number represents the number of protons found in the nucleus of an atom, and it is this defining characteristic that distinguishes one element from another on the periodic table. For example, all carbon atoms have an atomic number of 6, meaning they each contain six protons.

While options related to neutrons, electron configuration, and mass number might vary among atoms of the same element, the atomic number remains constant. The number of neutrons can differ, leading to different isotopes of the same element, and electron configurations can vary based on the atom's ionization. The mass number, which is the sum of protons and neutrons, will also change with different isotopes. Thus, it is the atomic number that serves as the primary identifier for the element itself.