How to read the periodic table
Each row is a period — atoms in the same period have electrons in the same outermost shell. Each column is a group — atoms in the same group share similar chemical behaviour because they have the same number of valence (outermost) electrons.
The two long rows below the main table are the lanthanides (57–71) and actinides (89–103). They're shown separately to keep the main grid compact; logically they slot into period 6 and 7 between groups 2 and 3.
Element categories
- Alkali metals (group 1, except H) — soft, very reactive, react vigorously with water.
- Alkaline earth metals (group 2) — reactive, but less so than alkali metals.
- Transition metals (groups 3–12) — typical metals; many form coloured compounds.
- Post-transition metals — metallic elements after the transition block (Al, Sn, Pb, etc.).
- Metalloids — properties between metals and nonmetals (Si, As, etc.); used in semiconductors.
- Reactive nonmetals — H, C, N, O, P, S, Se. Form most of the molecules of life.
- Halogens (group 17) — very reactive nonmetals; form salts with metals.
- Noble gases (group 18) — extremely unreactive; full outer shells.
- Lanthanides & actinides — inner-transition elements; many are radioactive (all actinides are).
FAQ
Why is hydrogen above the alkali metals?
Hydrogen has one valence electron like the alkali metals, but it's a nonmetal at standard conditions and behaves very differently. Its placement at the top of group 1 reflects the electron count, not chemical similarity. Some periodic tables float it above group 17 instead, or in its own column.
Are there elements beyond 118?
None confirmed. Elements 119 and 120 are being attempted in particle accelerators in Russia, the US, and Japan, but no successful synthesis has been verified as of 2024. Theoretically the periodic table could continue, but stability drops off rapidly.
Where do the atomic masses come from?
For naturally occurring elements, the listed mass is the standard atomic weight (a weighted average over natural isotope abundance), per IUPAC. For synthetic elements (no stable isotope), the mass shown is the atomic mass of the longest-lived known isotope, in parentheses by convention.