Quick Summary
Periodic classification arranges elements in a systematic order. This chapter helps you predict properties of elements using their position in the periodic table.
Key Definitions
- Periodic table: Arrangement of elements based on atomic number and recurring properties.
- Modern periodic law: Properties of elements are periodic functions of atomic numbers.
- Group: Vertical column in the periodic table.
- Period: Horizontal row in the periodic table.
- Valence electrons: Electrons present in the outermost shell.
- Atomic radius: Distance from the nucleus to the outermost shell of an atom.
Why Elements Are Classified
Classification makes the study of elements easier. If an element's group and period are known, we can predict its valency, metallic nature, atomic size, and chemical behaviour.
Periodic Trends
| Property | Across a period | Down a group |
|---|---|---|
| Atomic size | Decreases | Increases |
| Metallic character | Decreases | Increases |
| Non-metallic character | Increases | Decreases |
| Valency | Changes with valence electrons | Usually similar in a group |
Groups and Periods
Elements in the same group have similar chemical properties because they have the same number of valence electrons. Elements in the same period have the same number of electron shells.
Important Families
- Alkali metals: Group 1 elements, highly reactive metals.
- Alkaline earth metals: Group 2 elements.
- Halogens: Group 17 elements, reactive non-metals.
- Noble gases: Group 18 elements, generally inert due to stable electronic configuration.
Why the Trends Happen (reasoning)
- Across a period (left→right): the number of shells stays the same but nuclear charge (protons) increases, so electrons are pulled in tighter → atomic size decreases, and the atom holds electrons more strongly → metallic character decreases, non-metallic character increases.
- Down a group (top→bottom): a new shell is added each period, so the outer electrons are farther from the nucleus → atomic size increases, electrons are lost easily → metallic character increases.
Worked Examples
Example 1 — Position from configuration
An element has configuration 2, 8, 1. Find its group and period.
Example 2 — Predict valency
Element with configuration 2, 8, 7.
Example 3 — Compare atomic size
Which is larger: Na or Cl (both in period 3)?
Example 4 — Compare metallic character
Li, Na, K (Group 1) — order of increasing metallic character.
Textbook Solution Strategy
1. Find the atomic number or electronic configuration. 2. Identify valence electrons and number of shells. 3. Use valence electrons to find group trends. 4. Use number of shells to understand period and atomic size. 5. Connect the position to metallic or non-metallic character.
Important Questions
One Mark
- State the modern periodic law.
- What is a group?
- Name the elements of Group 18.
Two Marks
- Why do elements in the same group show similar properties?
- How does atomic size vary across a period?
- Differentiate between group and period.
Five Marks
- Explain periodic trends in atomic size and metallic character.
- Describe the importance of the modern periodic table.
- Predict the nature of an element from its electronic configuration.
Common Mistakes
- Confusing atomic mass with atomic number in the modern periodic table.
- Saying atomic size increases across a period.
- Forgetting that noble gases are in Group 18.
- Mixing up group number and period number.
Revision Checklist
- Modern periodic law memorised
- Group and period meanings revised
- Atomic size trend practised
- Metallic and non-metallic trends revised
- Important groups and families listed once from memory