Does #"atomic number"# determine chemistry or does #"Lewis Structure"#?

The atom is conceived to be composed of 3 fundamental particles: (i) electrons, negatively charged particles of negligible mass conceived to whizz about a massive nuclear core; (ii) protons, massive, positively charged, nuclear particles; and (iii) neutrons, massive neutrally charged nuclear particles. The massive particles compose the nuclear core.

The number of positively charged nuclear particles, the number of protons, gives #Z#, the so-called atomic number, and this number defines elemental identity. #Z=1#, the element is hydrogen; #Z=2#, the element is helium; #Z=3#, the element is lithium; #Z=4#, the element is beryllium.................

To raise another point of common misunderstanding, protons and electrons simply have OPPOSITE electronic charges. Our labels of positive and negative charges are simply arbitrary, and it would have made a lot more sense to assign the electron a positive charge, with a negative necessarily assigned to the proton.

Now in the nucleus, there can be a differing number of neutrons; these neutrons and protons engage in the strong nuclear force, whose explanation is well beyond my competence. The presence of different numbers of neutrons for a given atomic number, explains the existence of isotopes, and the quoted atomic mass is the average of the individual isotopic masses. The presence of positively charged particles in the nuclear core demands the presence of negatively charged particles outside the nuclear core, and these particles are called electrons. For a neutral atom, clearly there must be an equal number of electrons and protons.

The electrons are arranged around the nuclear core in specific levels and orbits. However, it is the outermost electrons, the so-called #"valence electrons"# that determine chemistry (it only took me half a page to get to the point of the question!)

Now you know that the Periodic Table reflects electronic structure. The Group 1 elements, i.e. the first vertical COLUMN in the Periodic Table, have the one valence electron, whose exchange and sharing with other elements gives rise to chemical bonds. Group 2 elements have 2 valence electrons. As #Z# increases, the nuclear charge increases, and the electrons that add to the outermost shell, the valence shell, may be removed upon oxidation, or upon reduction we conceive that more electrons to fill the valence shell.

To finally address your question, it is atomic number that determines chemistry. Mass determines the identity of the isotope.