Elements are arranged by atomic number. Patterns in chemistry follow electron-shell filling — that's why columns share properties.
Quick check
Why do elements in the same column of the periodic table behave so similarly chemically?
Reading the grid
- Rows (periods) — one row per electron shell being filled.
- Columns (groups) — same number of outer-shell electrons; similar reactions.
- Left side — metals (give electrons away easily).
- Right side — non-metals; the far-right column (noble gases) has full outer shells and barely reacts.
Mendeleev (1869) left gaps for elements not yet discovered and predicted their properties from the pattern. Gallium and germanium turned up later, almost exactly as he'd described — a triumph for the table.
Your turn
Sodium (group 1) reacts violently with water; argon (group 18) does nothing. What's different about their outer shells?