Chemistry
Periodic table of the elements
All 118 elements, searchable by name, symbol or atomic number. Tap a cell to open the full record: mass, electron configuration, electronegativity, density, melting and boiling points, ionisation energy, atomic radius, oxidation states and year of discovery.
How to read the periodic table
The elements are ordered by increasing atomic number — by proton count — and not by mass: the ordering Moseley showed to be the right one in 1913, and which settles the handful of pairs — argon and potassium, cobalt and nickel, tellurium and iodine — where mass would say the opposite. The rows are periods and count the occupied energy levels; the columns are groups and gather elements with the same electrons in their outermost level, and so with the same chemical behaviour.
Every periodic property follows from that arrangement. Down a group the atomic radius grows, because levels are being added, and electronegativity falls; across a period from left to right the radius contracts, because nuclear charge grows without adding a level, and electronegativity rises as far as fluorine, the most electronegative element there is. Ionisation energy runs opposite to radius: the further out and the better shielded an electron is, the less energy it takes to pull it off.
The two detached rows at the bottom are not a typographic whim: the lanthanides and actinides fill the f orbitals and, in their proper place, would make the table thirty-two columns wide instead of eighteen. For the same reason they carry no group number. The last elements of the seventh row have been made a few atoms at a time and many of their properties have never been measured: where the figure is missing, the field here is left empty rather than filled with a prediction.
Common mistakes
- Confusing atomic number with atomic mass: the first counts protons and is a whole number, the second is a weighted average over the natural isotopes and almost never is.
- Assuming ionisation energy always falls down a group: caesium is the easiest element of all to ionise, and francium below it goes back up, through relativistic effects.
- Looking for hydrogen among the alkali metals because it sits in the first column: it has one electron as they do, but it is a nonmetal and behaves nothing like them.
- Reading gas densities as though they were solid ones: here they are measured at 0 °C and one atmosphere, and hydrogen's is ninety thousand times smaller than gold's.
Frequently asked questions
Why are the elements ordered by atomic number rather than mass?
Because it is the proton count that decides chemical behaviour. Mendeleev ordered by mass and had to swap a few to make the families work; Moseley measured nuclear charge directly in 1913 and showed those swaps were the right order all along. Argon weighs more than potassium but has one proton fewer, and comes first.
What are the s, p, d and f blocks?
They say which kind of orbital the last electron goes into. The s block is the first two columns, the p block the last six, the d block the transition metals in between, the f block the two detached rows. An element's place in the table and its electron configuration are the same information written two ways.
Why do the lanthanides and actinides sit below the table?
Purely for printing. They belong to the sixth and seventh periods and should be inserted between group 3 and group 4, but that would make the table thirty-two columns wide. Detaching them keeps it readable on a page.
Why are some fields empty?
Because that figure has never been measured. The elements from 104 up exist for fractions of a second and in quantities of a few atoms: a density or a melting point can be predicted by calculation, but a prediction is not a measurement, and it is not written here as though it were.
How many elements occur in nature?
Ninety-four, as far as plutonium, though the last of them are found only in traces. Technetium and promethium, despite their low atomic numbers, have no stable isotopes and are vanishingly rare in nature. From 95 on, all are made artificially.
The properties shown, and where they come from
Atomic masses are the IUPAC conventional atomic weights; for elements with no stable isotope it is the mass number of the longest-lived one, which is what the textbook prints. Electronegativity is on the Pauling scale. Densities are in g/cm³, measured at room temperature for solids and liquids and at 0 °C and one atmosphere for gases. Melting and boiling points are in kelvin, first ionisation energy in kJ/mol, atomic radius in picometres. The electron configuration is written in shorthand, from the preceding noble gas.