Chemical categories
Mendeleev (1869) arranged the elements mainly by atomic mass and noticed that properties recurred — so clearly that he left gapsand predicted the properties of elements not yet discovered (his "eka-silicon" turned out to be germanium). Moseley (1913) used X-ray spectra to establish that the correct ordering criterion is the atomic number , the number of protons; nuclear charge is . This is the basis of the modern periodic law:
In chemistry, "periodic" means that as you move through certain configurations and properties recur. It is not a mathematical periodic function with a fixed interval: the periods contain 2, 8, 8, 18, 18, 32 and 32 elements. Each main row represents one period, with the f block detached for convenience.
The recurrence has an electronic cause. Each period begins when a new principal level starts to fill, although and subshells may also fill along the way. Among representative elements, a group shares a valence pattern and therefore similar chemistry: alkali metals end in and tend to lose one electron; halogens end in; and noble gases have closed shells, although the heavier ones can form compounds.
The s, p, d and f blocks identify the subshell receiving the differentiating electron. The f block is the reason for the two detached rows at the bottom: drawn in their true place, lanthanides and actinides would stretch the table to 32 columns. That is what the 57–71 and 89–103 markers in column 3 are for — click them to highlight how those series fit into the long form. The exact assignment of group 3 varies among published tables.
Almost every trend follows from a single idea: the effective nuclear charge , the charge an outer electron actually "feels" after the inner electrons screen the nucleus. Moving right across a period adds protons while the shell stays the same, so grows; moving down adds a whole new shell, leaving the outer electron farther away and better screened.
All three quantities of the classic triad can be colored on the table from "Color by → Property":
NIST reports ionization energy per atom, in eV; the table expresses it per mole of atoms by multiplying by the Avogadro constant and the elementary charge, both exact in the SI:
When a standard atomic weight exists, the tabulated value is the weighted average of isotopes in terrestrial materials by abundance :
Calculate a specific composition with Average Atomic Mass and Isotopes.
That is why chlorine "weighs" 35.45 u: it is ~76% and ~24% , not an isotope of mass 35.45. And when you see a mass in brackets — [97] for Tc, [223] for Fr or [294] for Og — the element has no standard atomic weight. Following IUPAC, the table shows the mass number of the nuclide with the longest confirmed half-life; this does not simply depend on whether the element occurs naturally in trace amounts.
At 25 °C and 1 atm only two elements are liquid: Hg and Br. But gallium melts at 29.8 °C and cesium at 28.4 °C. The "State at T" mode uses a simplified 1 atm model, not a complete phase diagram:
At an exact transition point, phases coexist; by convention the visualization shows the higher-temperature phase. Three elements do not follow that scheme at 1 atm. Helium does not solidify by cooling: its "melting point" of −272.2 °C requires about 25 atm. Arsenic sublimes at 616 °C and carbon (graphite) at 3825 °C; their melting points, 817 °C and 4489 °C, are triple points at 3.7 and 10.3 MPa. That is why those values stay out of the melting and boiling maps. The model also omits plasma, ionization and allotrope transitions.
Density is colored by quantiles: each color band holds the same number of elements. With 5 orders of magnitude between gaseous H and osmium, a linear or logarithmic scale would leave nearly every solid the same blue. Gas densities are converted from standard-condition values; the map compares ranges, not measurements at a single temperature.
Gray means missing data. The ≈ symbol marks theoretical, estimated or extrapolated values: properties of superheavy elements, of astatine and francium (no visible sample has ever been made), covalent radii from berkelium onward, ionization energies that NIST lists as semi-empirical or calculated, electronegativities that the compilations give only as a range or a generic value, and electron configurations from rutherfordium onward, which come from calculations. Ionization energies show only the digits their uncertainty supports: dubnium, at ±0.5 eV, appears as 660 kJ/mol.
The atomic masses in this table are the ones Advanced Stoichiometry uses to compute molar masses. The oxidation states on each card are the starting point for redox balancing by half-reactions. And electronegativity anticipates the kind of bonding you will see in those compounds.