Interactive explorer of all 118 elements in the real IUPAC layout
The modern periodic table is the outcome of a major historical transition. Mendeleev arranged elements by relative atomic mass and repeating valence patterns, leaving gaps for undiscovered species. Later, Moseley demonstrated that the physically correct ordering variable is the atomic number (Z), defined by nuclear charge. Therefore, periodicity is interpreted as the recurrence of chemical and physical properties as a function of Z, not mass.
Each period is linked to the dominant principal energy level (n) of valence electrons. Elements in the same group or family share similar outer electronic configurations, which explains comparable reactivity, common oxidation states, and bonding behavior. The s, p, d, and f blocks correspond to the subshell being populated according to the Aufbau principle.
Tabulated atomic mass is a weighted isotopic average expressed in unified atomic mass units (u).Electronegativity, typically reported on the Pauling scale, quantifies an atom's relative tendency to attract electron density within a covalent bond.Oxidation states are formal charges that support electron-transfer analysis, compound formulation, and redox balancing.
Atomic radius generally decreases from left to right due to increasing effective nuclear charge, and increases down a group as additional shells are occupied.Ionization energy tends to vary in the opposite direction: it rises across a period and usually declines down a family.Electron affinity, despite important exceptions (half-filled and filled subshell effects), is typically more favorable toward the right side and less favorable down a group.
Technical reading note: university-level interpretation should combine trends with electron configuration, shielding, and effective nuclear charge, instead of relying on memorized rules in isolation.