atomic weight periodic table
The atomic weight shown on the table is not the mass of a single atom: it is a weighted average across the natural abundance of each isotope of the element. That is why chlorine shows up as an interval near 35.45 instead of a whole number, even though individual protons and neutrons each have an almost whole-number mass.
Two stable isotopes, chlorine-35 (~75.77%) and chlorine-37 (~24.23%), weighted by abundance land in this range near 35.45; no real atom weighs exactly that value.
Carbon-12 defines the atomic mass unit itself (1u = 1/12 of its mass), but natural samples vary slightly because of carbon-13, which is why the table shows an interval instead of a single value.
With no stable isotope in nature, there is no abundance to average; that is why the atomic weight cell stays empty for this element, unlike most of the other 117 elements.
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).
Because it is a weighted average between two stable isotopes, chlorine-35 and chlorine-37, each with its own natural abundance. Weighting about 75.77% chlorine-35 and 24.23% chlorine-37 lands the average near 35.45, the reference value shown, even though no single atom carries that exact mass.
No. Mass number is always a whole number: the sum of protons and neutrons of one specific isotope, like the 12 nucleons of carbon-12. Atomic weight on the periodic table is the weighted average across all natural isotopes of an element, which is why it is rarely a whole number.
u (unified atomic mass unit) and Da (dalton) are the same unit under different names. By definition, 1 u equals 1/12 the mass of a carbon-12 atom, about 1.6605 × 10⁻²⁷ kg, and that is the reference used for every atomic weight value shown on the table.
The table runs locally in your browser and does not send searches to a server.