Chemical Period
A chemical period is a horizontal row in the periodic table, in which all elements have the same number of electron shells (principal quantum levels). There are seven periods in the modern periodic table, corresponding to the seven principal energy levels occupied by electrons in known elements. Moving across a period from left to right, the atomic number increases by one with each step, leading to systematic changes in properties such as metallic character, atomic radius, and ionisation energy.
Summary of the seven periods in the periodic table
| Period | Elements | Electron Shells | Starts With | Ends With |
|---|---|---|---|---|
| 1 | 2 | 1 | Hydrogen (H) | Helium (He) |
| 2 | 8 | 2 | Lithium (Li) | Neon (Ne) |
| 3 | 8 | 3 | Sodium (Na) | Argon (Ar) |
| 4 | 18 | 4 | Potassium (K) | Krypton (Kr) |
| 5 | 18 | 5 | Rubidium (Rb) | Xenon (Xe) |
| 6 | 32 | 6 | Caesium (Cs) | Radon (Rn) |
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Related Terms
Periodic Table
The periodic table is a tabular arrangement of all known chemical elements ordered by increasing atomic number, with elements having similar chemical properties placed in vertical columns called groups. It was developed by Dmitri Mendeleev in 1869 and serves as the foundational reference for all of chemistry. The table reveals periodic trends in elemental properties such as atomic radius, ionisation energy, and electronegativity, enabling scientists to predict the behaviour of elements and their compounds.
Chemical Group
A chemical group (also called a family) is a vertical column in the periodic table containing elements that share the same number of valence electrons, and therefore exhibit similar chemical properties and reactivity patterns. The modern IUPAC system numbers groups 1 through 18 from left to right. Elements within a group show predictable trends: for example, reactivity increases down Group 1 (alkali metals) because the outermost electron is progressively further from the nucleus and more easily lost.
Periodic Trend
Periodic trends are systematic patterns in elemental properties that arise from the regular variation in nuclear charge and electron configuration across periods and down groups of the periodic table. Key periodic trends include atomic radius, ionisation energy, electron affinity, electronegativity, and metallic character, all of which change predictably as atomic number increases. Understanding periodic trends allows chemists to predict chemical reactivity, bond types, and physical properties of elements and their compounds without needing to memorise individual data for every element.
From Greek "periodos" meaning a recurring cycle or circuit. In chemistry, "period" was adopted because elemental properties repeat (recur) in a regular pattern as atomic number increases — every new row starts a new cycle of electron shell filling.