---
tags:
  - science/chemistry
  - periodic-table
created: 2026-05-16
breadcrumb: "Chemistry > Periodic table > Periodic table essentials"
---

# Periodic table essentials

> **Mechanism:** Periodic recurrence of properties with atomic number

## Items

- **Group** — Column — atoms with similar outer-shell electron count and chemistry.
- **Period** — Row — atoms with the same number of electron shells.
- **Alkali metals** — Group 1; one outer electron; reactive, soft.
- **Halogens** — Group 17; one electron short of full outer shell; very reactive.
- **Noble gases** — Group 18; full outer shell; chemically inert.
- **Transition metals** — d-block; variable oxidation states; coloured compounds.
- **Lanthanides / actinides** — f-block; rare-earth and heavy radioactive elements.

## Related

- [[atoms-bonding]]