atomic number periodic table
Atomic number (Z) is the count of protons in the nucleus of an atom. It alone decides which element is which, from hydrogen (Z=1) to oganesson (Z=118), which is why the periodic table is ordered by increasing Z instead of by atom mass.
Iron has natural isotopes with 54, 56, 57 and 58 nucleons, a different mass number in each, but Z=26 fixed in every one: that is what keeps all four as iron.
Technetium was the first synthetic element discovered and has no stable isotope in nature; Z=43 still defines the element even with no reference mass to average.
Z=118 marks the current edge of the table: no element with a higher atomic number has been confirmed by IUPAC to date.
Atomic number is the number of protons in an atom's nucleus, and it's what defines the element's identity. Every atom with 6 protons is carbon, with 8 protons is oxygen, with 26 protons is iron, and so on. That's why the table is ordered by this number, from 1 (hydrogen) to 118 (oganesson).
No. Atomic number (Z) counts only protons and never changes for a given element: carbon is always Z=6. Atomic mass adds protons and neutrons and varies between isotopes of the same element, which is why carbon-12 and carbon-14 share Z but have different masses.
No, by definition. Changing the proton count already makes the atom a different element: one proton less than carbon (Z=6) gives boron (Z=5), one more gives nitrogen (Z=7). Only the neutron count can vary without changing the element, which is what creates isotopes.
118, from hydrogen (Z=1) to oganesson (Z=118). The last four, numbers 113, 115, 117 and 118, were officially named by IUPAC in November 2016, completing the seventh row of the periodic table.
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