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Elements are arranged by atomic number (the number of protons) in 7 periods (rows) and 18 groups (columns). Elements in the same group have similar outer-electron arrangements, which is why they behave alike: the alkali metals in group 1 all react strongly with water, and the noble gases in group 18 barely react at all. The two rows below the main table hold the lanthanides and actinides. They belong in periods 6 and 7 but are pulled out so the table fits on a page.
Atomic weight is the standard atomic weight, the average mass of an element's atoms as found in nature, in daltons (atomic mass units). Some values are given with fewer digits because natural samples vary; hydrogen, lithium, carbon and several others differ slightly depending on where they come from. Elements with no stable isotopes have no standard atomic weight, so the table shows the mass number of the longest-lived known isotope in brackets, such as [98] for technetium. For the heaviest elements these bracketed numbers can change as longer-lived isotopes are discovered, and reference tables don't always agree.
Electron configurations use the noble-gas shorthand: [Ne] 3s2 3p4 for sulfur means the neon core plus six more electrons. They follow the usual filling order (1s, 2s, 2p, 3s, 3p, 4s, 3d …) with the well-known exceptions such as chromium and copper, which move an electron into the d subshell. Configurations for elements 104 and above have not been measured and are theoretical predictions. The block (s, p, d or f) names the subshell being filled across that part of the table. Lanthanum and actinium are listed as d-block here, though some tables treat them as f-block.
Example: iron (Fe) is element 26 in period 4, group 8. Its standard atomic weight is 55.845, it is a transition metal in the d-block, and its configuration is [Ar] 3d6 4s2.
What is the difference between atomic number and atomic weight?
The atomic number counts the protons in an atom and defines the element: every carbon atom has 6. Atomic weight is the average mass of the atoms, which also depends on neutrons and on the mix of isotopes found in nature, so it is not a whole number.
Why do some weights have brackets?
Elements such as technetium, promethium and everything above uranium (plus a few others) have no stable isotopes and no consistent natural mix, so there is no standard atomic weight. The bracketed number is the mass number of the longest-lived isotope known.
Where do the lanthanides and actinides go?
They fit between groups 2 and 3 in periods 6 and 7. Placing them inline would make the table 32 columns wide, so they are shown in two rows underneath, linked by the 57–71 and 89–103 markers.
Which elements are metalloids?
This table marks boron, silicon, germanium, arsenic, antimony and tellurium as metalloids. Classification of borderline elements like polonium and astatine varies between textbooks.
Can I use this table offline?
Once the page has loaded, the table works without an internet connection because all the data is part of the page.