AllChemicals — The Online Chemical Glossary | https://allchemicals.info/chemical/669-period
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Quick Reference
Also Known As
Horizontal row (periodic table), row, period number, shell number (related)
What is Period?
A horizontal row in the periodic table. Elements in the same period have the same number of electron shells (principal quantum number n). Properties change systematically across a period as electrons are added to the same shell with increasing nuclear charge.
Properties & Characteristics
A period is a horizontal row in the periodic table. Elements in the same period have the same highest principal quantum number (n) for their outermost electrons. Period 1 has 2 elements (H, He); Period 2 has 8 (Li to Ne); Period 3 has 8 (Na to Ar); Periods 4 and 5 have 18 (include transition metals); Periods 6 and 7 have 32 (include lanthanides/actinides). Physical and chemical properties change systematically across a period as atomic number increases.
Uses & Applications
Period relationships explain periodic trends: across any period, atomic radius decreases, electronegativity increases, ionisation energy generally increases, and character changes from metallic to metalloid to non-metallic. These trends allow prediction of properties of elements and their compounds.
Safety Information
Within a period, hazard varies from very reactive metals on the left (Na, K — react with water) to reactive nonmetals on the right (Cl, F — toxic, oxidising), with relatively unreactive noble gases at the far right. Period position gives a rough guide to reactivity and hazard type.
Always consult the SDS/MSDS before handling any chemical. This information is for educational purposes only.
SynonymsHorizontal row (periodic table), row, period number, shell number (related)
Frequently Asked Questions
A horizontal row in the periodic table. Elements in the same period have the same number of electron shells (principal quantum number n). Properties change systematically across a period as electrons are added to the same shell with increasing nuclear charge.
Period relationships explain periodic trends: across any period, atomic radius decreases, electronegativity increases, ionisation energy generally increases, and character changes from metallic to metalloid to non-metallic. These trends allow prediction of properties of elements and their compounds.
Within a period, hazard varies from very reactive metals on the left (Na, K — react with water) to reactive nonmetals on the right (Cl, F — toxic, oxidising), with relatively unreactive noble gases at the far right. Period position gives a rough guide to reactivity and hazard type.
Editorial standards: Chemical data is sourced from peer-reviewed literature,
CAS Registry, NIST WebBook, and PubChem. Safety information reflects guidance from OSHA, ECHA,
and IAEA. For educational purposes only — always consult official SDS documentation and qualified
professionals before handling chemicals.
