Column logic
Groups run vertically. Elements sharing a column share valence-shell occupancy, which is why group 17 corrodes and group 18 refuses to react at all.
Row logic
Periods run horizontally. Each new row opens a new principal shell; atomic radius contracts left-to-right as nuclear charge tightens the grip.
Detached blocks
The lanthanide and actinide series are pulled out beneath the main body so the f-block does not stretch the table to 32 columns.
Calibration
Masses are IUPAC 2021 conventional values. Bracketed figures denote the most stable known isotope of a synthetic element.