A wall chart gives you a symbol and a number. The interesting part is what the arrangement is for: why argon closes its shell at eight and not eighteen, why atoms get smaller as you move right, and why the table is ordered by proton count rather than by weight — which it had to be, because in four places the weights run backwards.
The gap between group 2 and group 13 in periods 4 and 5 is real: it is where the d-block sits. The two rows underneath are pulled out only so the page fits on a screen — they belong inside period 6 and 7 at group 3.
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A density means nothing on its own. Each bar is where this element sits across every element that has a value, so “dense” becomes a position.
The table shows you the shape of the thing; a list lets you rank it. Tap a column to sort. Totals across all 118 are at the bottom.
The weights run backwards in four places. Argon is heavier than potassium, cobalt than nickel, tellurium than iodine — and thorium than protactinium, which is the one almost nobody lists. Mendeleev ordered by weight and had to swap pairs to make the chemistry work; the table is ordered by proton count because that is the thing that actually decides how an element behaves. Three further apparent inversions among the synthetic elements are not real: they come from comparing a measured atomic weight against the mass number of a longest-lived isotope, which are two different quantities.
Argon closes at eight, not eighteen. The third shell can hold eighteen electrons, so the obvious question is why argon is inert with only eight in it. Because the 4s subshell is lower in energy than 3d: potassium starts a whole new shell rather than continuing to fill argon's. That single fact is why the transition metals exist as a block at all.
Atoms get smaller as you go right. Adding electrons ought to make an atom bigger and it does the opposite — across period 2, lithium is 167 pm and fluorine 42. Each step right adds a proton to the nucleus without adding a shell, so the same shell is pulled in harder. Go down a group and a new shell is added, so the atoms grow.
Hafnium is the same size as zirconium. Fifteen lanthanides sit between them, each adding a proton, and the f-electrons shield the pull poorly — so the atoms contract across the series and hafnium comes out at 208 pm against zirconium's 206, a whole extra electron shell for a one per cent difference. It is why the two are chemically so alike that hafnium went undiscovered inside zirconium ores until 1923.
Palladium has an empty outer shell. Every element in the table occupies as many shells as its period number, except palladium: it is [Kr]4d10 with nothing in 5s, so it fills four shells while sitting in period five. One element, one exception.
Arsenic never melts. Its quoted melting point of 1090 K is above its boiling point of 887 K, which looks like a data error and is not: at ordinary pressure arsenic sublimes, going straight from solid to vapour. It is the only element here that does.
Standard atomic weights where an element has one, and the mass number of the longest-lived isotope where it does not — the two are not the same quantity and the page marks which is which. Densities are at standard temperature and pressure, so gases look like rounding errors beside metals. Radii are empirical values and vary by up to a few per cent between published sets; use them for the trend, not for a calculation. Electronegativity is the Pauling scale, which has no value for most noble gases. Elements above 100 have properties that are predicted rather than measured, and the page says so. Not modelled: isotopes, ions, allotropes — carbon as graphite and as diamond are one entry here and two very different materials in reality.
The space between groups 2 and 13 is not formatting. It is where the d-block goes, and the table's shape is a picture of the order shells fill in.
Elements in a group share an outer-shell count, so they behave alike. That is what let Mendeleev leave holes and describe elements nobody had found yet.
Osmium at 22.59 g/cm³ means little until you see it is the top of all 92 natural elements, and that lithium floats on water at 0.53.
Non-metals are a small corner at the top right. The intuition that elements are mostly gases and powders comes from which ones you meet, not from how many there are.
Releases in which this page changed, newest last. Derived from the archived copy of every release, not from notes written afterwards — so it reflects what actually shipped. Site-wide passes are left out; they are in the full changelog.