How to use it
- Type a name, a symbol or an atomic number in Search. Elements that do not match fade out.
- Choose what the colours show in Colour by: the block, the group, or a property such as the atomic mass.
- Click an element, or move to it with the arrow keys and press Enter, to read its card below the table.
- Read the standard atomic weight with its uncertainty. An element with no standard atomic weight shows the mass of one of its isotopes.
How the table is arranged
The table has 18 columns, the groups, and 7 rows, the periods. Elements in the same group behave alike in chemistry. The lanthanides and actinides belong in periods 6 and 7, and they are set in two rows below the main table so that it stays narrow.
The block of an element is the subshell that its outermost electrons fill: s for groups 1 and 2, p for groups 13 to 18, d for groups 3 to 12, and f for the lanthanides and actinides. Helium is an s-block element here, because its two electrons are in the 1s subshell.
Where the atomic weights and properties come from
The standard atomic weights are the CIAAW abridged values of 2024, with five significant figures and their uncertainty. They apply to normal materials; a sample with an unusual isotope mix can differ in the last digits.
An element with no stable isotope in normal material, such as technetium or promethium, has no standard atomic weight. Its card shows the mass of one isotope instead, and the values for elements 104 to 118 are predicted. Electronegativity, melting and boiling points, density and the year of discovery are as PubChem gives them, with temperatures in kelvin and density in g/cm³. The one exception is the year for aluminium and calcium: PubChem lists them as known since antiquity, so the card gives the year of first isolation from the Wikipedia timeline of discoveries, 1825 and 1808. Only the elements the timeline lists as known in ancient times show "known since antiquity".
What the card does not show
- A value that is not known, or that does not exist for the element, shows as No data. For example, the noble gases have no electronegativity on the page.
- The card does not show isotopes, ionisation energies or oxidation states.
Worked example: oxygen
Oxygen is element 8. It is in period 2, group 16, and the p-block. Its standard atomic weight is 15.999 ± 0.001, its electron configuration is [He]2s² 2p⁴, and its electronegativity is 3.44 on the Pauling scale.
| Property | Oxygen |
|---|---|
| Atomic number | 8 |
| Standard atomic weight | 15.999 ± 0.001 |
| Group and period | 16 and 2 |
| Block | p-block |
| Electron configuration | [He]2s² 2p⁴ |
| Electronegativity | 3.44 |
Frequently asked questions
Why are the lanthanides and actinides shown below the table?
The main table would otherwise be far too wide. The lanthanides belong in period 6 and the actinides in period 7, in group 3 between barium and hafnium and between radium and rutherfordium. The two rows show their 15 elements each.
What does the block tell me?
The block shows which subshell the outer electrons fill, and elements in the same block tend to behave alike. The s-block holds the alkali and alkaline-earth metals of groups 1 and 2, and the p-block holds the halogens and the noble gases.
Why does some element have no standard atomic weight?
Technetium and promethium have no stable isotope in normal material, so they have no standard atomic weight. The card shows the mass of one isotope instead, which is only an approximation for a normal sample.
What is electronegativity?
It is a measure of how strongly an atom draws shared electrons towards itself. The values here are on the Pauling scale, and fluorine has the highest of them, 3.98.
What does the bracket in an electron configuration mean?
A bracket such as [Ar] stands for the configuration of the noble gas before the element. For iron, [Ar]4s² 3d⁶ means the argon core plus 4s² and 3d⁶.
Does the page send my search or my choices anywhere?
No. The table, the search and the colouring run in your browser, and nothing you type is stored or sent to our server.
Sources
- CIAAW: Abridged Standard Atomic Weights 2024
- PubChem periodic table data (NCBI, US government)
- Wikipedia: Timeline of chemical element discoveries
- Turkish names: Wikipedia (Turkish), article Element
Figures and text last checked on Wednesday, November 11, 2026.