Periodic Table
Summary: The Periodic Table arranges all known elements in order of increasing atomic number (proton number). Elements are organised into horizontal rows called periods and vertical columns called groups. Elements in the same group have the same number of outer shell electrons and similar chemical properties. Tags: igcse chemistry definition periodic-table Created: 2026-07-14 Last Updated: 2026-07-23
The Periodic Table is a systematic arrangement of all known chemical elements in order of increasing atomic number (proton number). It is the single most important organisational tool in chemistry.
Arrangement of Elements
Periods
- Horizontal rows in the Periodic Table
- Period number = number of occupied electron shells in the atom
- There are 7 periods (only Periods 1–3 are covered in detail at IGCSE)
Groups
- Vertical columns in the Periodic Table
- Group number = number of electrons in the outer shell (for Groups 1–2 and 13–18)
- Elements in the same group have the same number of outer shell electrons and therefore similar chemical properties
Metallic to Non-Metallic Character Across a Period
Moving from left to right across a period:
- Metallic character decreases
- Non-metallic character increases
- The left side contains reactive metals (Group 1)
- The right side contains non-metals (Group 7, Group 0)
- Elements transition from metals → metalloids → non-metals
Example — Period 3: Na (metal) → Mg (metal) → Al (metal) → Si (metalloid) → P (non-metal) → S (non-metal) → Cl (non-metal) → Ar (non-metal)
Group Number and Ion Charge
The charge on the ion formed by an element is related to its group number:
| Group | Outer Electrons | Ion Formed | Charge |
|---|---|---|---|
| 1 (Alkali metals) | 1 | M⁺ | +1 |
| 2 (Alkaline earth) | 2 | M²⁺ | +2 |
| 13 | 3 | M³⁺ | +3 |
| 14 | 4 | Varies | — |
| 15 | 5 | X³⁻ | −3 |
| 16 | 6 | X²⁻ | −2 |
| 17 (Halogens) | 7 | X⁻ | −1 |
| 18 (Noble gases) | 8 (full) | No ion formed | 0 |
Trend: Metals lose electrons to form positive ions; non-metals gain electrons to form negative ions. The ion charge is determined by how many electrons are lost or gained to achieve a noble gas configuration.
Predicting Element Properties from Position
An element’s position in the Periodic Table can be used to predict:
- Electronic configuration — number of electron shells and outer electrons
- Chemical properties — reactivity, type of bonding, type of oxide formed
- Physical properties — metallic/non-metallic character, melting point trends
- Ion formed — charge based on group number
Key Facts
- Elements arranged by increasing atomic number (proton number)
- Period = horizontal row = number of electron shells
- Group = vertical column = number of outer shell electrons
- Same group → same outer electrons → similar chemical properties
- Metallic character decreases across a period (left to right)
- Non-metallic character increases across a period
- Group number determines the charge on the ion formed
See Also
- Periodic Table Trends
- Group 1 Alkali Metals
- Group 7 Halogens
- Group 0 Noble Gases
- Transition Metals
- Metallic Character
- Group Number
- IGCSE-Chem-Index
This is a key term definition page — part of the IGCSE Chemistry (0620/0971) keyword glossary.