The periodic table is an arrangement of all known elements in order of increasing atomic number. Elements with similar properties are placed in the same group (vertical column).
Periods and Groups
- Periods are horizontal rows (1 to 7). Elements in the same period have the same number of electron shells.
- Groups are vertical columns (1 to 18, or I–VIII). Elements in the same group have the same number of outer (valence) electrons, so they have similar chemical properties.
Important Groups
| Group | Name | Examples |
|---|---|---|
| Group 1 | Alkali metals | Lithium, sodium, potassium |
| Group 2 | Alkaline earth metals | Magnesium, calcium |
| Group 7 | Halogens | Fluorine, chlorine, bromine |
| Group 8/0 | Noble gases | Helium, neon, argon |
Trends in the Periodic Table
- Across a period (left to right): atomic size decreases; metallic character decreases; non-metallic character increases.
- Down a group: atomic size increases; reactivity of alkali metals increases; reactivity of halogens decreases.
Valence electrons rule: Group number = number of outer electrons (for main groups). This determines an element’s valency and the type of ions it forms.
Uses of the Periodic Table
- Predicts the properties of elements.
- Shows relationships between elements.
- Helps predict the formulae of compounds.
Summary
The periodic table arranges elements by atomic number. Periods are rows, groups are columns. Elements in a group share chemical properties because they have the same number of outer electrons. Key families include alkali metals, halogens and noble gases.
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