The periodic table is commonly divided into four blocks, each grouping elements whose most recently added electron occupies the same type of atomic subshell. This structural division highlights recurring chemical patterns and links an element's position to its electron configuration. For an overview of the layout see the periodic table.

In quantum terms, blocks are defined by the character of the orbital that contains the element's highest-energy (outer) electron. That last electron is part of the atom's valence shell and belongs to an s, p, d, or f orbital. The concept emphasizes the role of electrons in determining chemical behavior; for background on the particles involved, see electrons, and for the orbitals themselves see atomic orbital.

Four conventional blocks

  • s-block: Groups 1–2 (and often helium). Elements with their highest electron in an s orbital; typically highly reactive metals like sodium or alkaline earth metals.
  • p-block: Groups 13–18. Contains metals, metalloids and nonmetals; includes familiar elements such as chlorine and carbon.
  • d-block (transition metals): Groups 3–12. Characterized by partially filled d orbitals; often show multiple oxidation states and metallic bonding (examples: iron, copper).
  • f-block (lanthanides and actinides): Inner-transition series usually shown below the main table; electrons enter f orbitals and give rise to complex magnetic and optical properties.

Blocks help explain trends such as valence behavior, ionic charges, and the occurrence of metallic or nonmetallic character. Certain placements remain subject to convention: hydrogen can be associated with the s-block but sometimes is discussed separately, and helium is sometimes shown with noble gases despite its s-shell configuration.

The block concept has been part of periodic-table theory for decades and was emphasized in alternative layouts such as Charles Janet’s work. While useful for organizing chemical behavior, blocks are a simplification of electron interactions and serve as a practical guide rather than an absolute classification.