Class 10 Science Chapter 5 Periodic Classification of Elements Notes
🔹 1. Periodic Table
The periodic table is a systematic arrangement of elements in which:
Elements with similar properties are placed in the same vertical column (group).
Elements are arranged in increasing order of atomic number.
🔹 2. Need for Classification of Elements
Large number of elements discovered
Difficult to study them individually
Classification helps in:
Easy study
Predicting properties of elements
Understanding trends in properties
🔹 3. Early Attempts at Classification
(a) Dobereiner’s Triads
Dobereiner grouped elements into groups of three with similar properties.
Characteristics
Properties of elements in a triad are similar
Atomic mass of middle element ≈ average of other two
Example
| Element | Atomic Mass |
|---|---|
| Li | 6.9 |
| Na | 23 |
| K | 39 |
➡️ 23 ≈ (6.9 + 39) / 2
Limitations
Only three triads could be identified
Failed to classify all known elements
(b) Newlands’ Law of Octaves
According to Newlands:
When elements are arranged in increasing order of atomic masses, every 8th element has properties similar to the first.
Merits
First systematic arrangement of elements
Limitations
Applicable only up to Calcium
No place for new elements
Dissimilar elements placed together
Assumed only 56 elements existed
🔹 4. Mendeleev’s Periodic Table
Mendeleev’s Periodic Law
“The physical and chemical properties of elements are the periodic function of their atomic masses.”
Features
Elements arranged in increasing atomic mass
Similar elements placed in same group
Groups → vertical columns
Periods → horizontal rows
Merits of Mendeleev’s Periodic Table
Left gaps for undiscovered elements
Predicted properties of:
Eka-boron
Eka-aluminium
Eka-silicon
Helped in correcting atomic masses
Noble gases accommodated later without disturbing table
Limitations of Mendeleev’s Periodic Table
No fixed position for Hydrogen
No place for isotopes
Atomic masses not in increasing order (Co–Ni anomaly)
No clear distinction between metals and non-metals
🔹 5. Modern Periodic Table
Modern Periodic Law
“The physical and chemical properties of elements are the periodic function of their atomic number.”
Basis
Atomic number (Z)
Electronic configuration
Structure of Modern Periodic Table
18 groups (vertical columns)
7 periods (horizontal rows)
Elements in same group → same valence electrons
Period number = number of shells
🔹 6. Trends in Modern Periodic Table
(i) Valency
Valency depends on number of valence electrons.
In a period:
Valency increases from 1 → 4, then decreases to 0In a group:
Valency remains same
(ii) Atomic Size (Atomic Radius)
Distance between nucleus and outermost shell.
Across a period:
Atomic size decreases
(Due to increase in nuclear charge)Down a group:
Atomic size increases
(Due to addition of new shells)
(iii) Metallic Character
Tendency to lose electrons.
Across a period: Decreases
Down a group: Increases
Example:
(iv) Non-Metallic Character
Tendency to gain electrons.
Across a period: Increases
Down a group: Decreases
Example:
(v) Chemical Reactivity
Metals
Reactivity increases down the group
Non-metals
Reactivity decreases down the group
(vi) Electronegativity
Tendency to attract shared electrons.
Across a period: Increases
Down a group: Decreases
(vii) Nature of Oxides
Metal oxides → Basic
Na₂O, MgO
Non-metal oxides → Acidic
SO₃, P₂O₅, Cl₂O₇
🔹 7. Groups and Periods
Group: Vertical columns
Period: Horizontal rows
| Feature | Group | Period |
|---|---|---|
| Valence electrons | Same | Change |
| Number of shells | Increase down | Same |
🔹 8. Comparison: Mendeleev vs Modern Periodic Table
| Mendeleev | Modern |
|---|---|
| Based on atomic mass | Based on atomic number |
| No place for isotopes | Isotopes placed together |
| Hydrogen position unclear | Clear trends |
| 8 groups | 18 groups |
✨ One-Line Revision Points
Atomic number is basis of modern periodic table
Properties repeat periodically
Size ↓ across period, ↑ down group
Metals on left, non-metals on right
📝 PYQs (Previous Years’ Questions)
🔹 1 Mark PYQs
What is the basis of the modern periodic table?
How many groups are there in the modern periodic table?
How many periods are present in the modern periodic table?
What is meant by atomic number?
Name the scientist who gave the modern periodic law.
What are isotopes?
Which property remains same in a group?
Which element was predicted as eka-aluminium by Mendeleev?
🔹 2 Marks PYQs
State Dobereiner’s law of triads.
What is Newlands’ law of octaves?
Write two merits of Mendeleev’s periodic table.
Write two limitations of Mendeleev’s periodic table.
Define modern periodic law.
What is meant by periodicity of properties?
Why atomic size decreases across a period?
🔹 3 Marks PYQs
Explain Dobereiner’s triads with one example.
State Newlands’ law of octaves and write its limitations.
Write any three merits of Mendeleev’s periodic table.
Explain variation of valency in a period and a group.
Explain trend of atomic size in the modern periodic table.
Why metallic character increases down a group?
🔹 5 Marks PYQs
Describe Mendeleev’s periodic table and its limitations.
Explain the modern periodic table on the basis of electronic configuration.
Compare Mendeleev’s periodic table with modern periodic table.
Explain trends in the modern periodic table (any three properties).
Describe Dobereiner’s triads and Newlands’ law of octaves.
Explain periodic trends in metallic and non-metallic character.
⭐ Very Important Repeated PYQs
✔ Why modern periodic table is better than Mendeleev’s periodic table?
✔ Why isotopes have same position in modern periodic table?
✔ Why atomic radius increases down a group?
✔ Why noble gases are placed in a separate group?
✔ Arrange the elements in increasing metallic character:
Na, Mg, Al
✍️ Top 10 Short Question–Answers
Q1. What is the periodic table?
Ans: The periodic table is a systematic arrangement of elements in increasing order of atomic number.
Q2. Why is classification of elements needed?
Ans: To make the study of elements easy and to understand trends in their properties.
Q3. What was the basis of Dobereiner’s triads?
Ans: Atomic mass and similarity in properties.
Q4. State Newlands’ law of octaves.
Ans: Every 8th element shows properties similar to the first when arranged by atomic mass.
Q5. State Mendeleev’s periodic law.
Ans: Properties of elements are periodic functions of their atomic masses.
Q6. What is the basis of the modern periodic table?
Ans: Atomic number.
Q7. How many groups and periods are there in the modern periodic table?
Ans: 18 groups and 7 periods.
Q8. What happens to atomic size across a period?
Ans: Atomic size decreases.
Q9. What is valency dependent on?
Ans: Number of valence electrons.
Q10. What is the nature of metal oxides?
Ans: Basic in nature.
📝 Long Answer Questions
Q1. Explain Dobereiner’s Triads and Newlands’ Law of Octaves.
Ans:
Dobereiner’s Triads:
Elements grouped in sets of three
Similar chemical properties
Atomic mass of middle element ≈ average of other two
Limitation: Could classify only few elements
Newlands’ Law of Octaves:
Every 8th element had similar properties
First systematic arrangement
Limitations: Valid only up to calcium, no space for new elements
Q2. Describe Mendeleev’s periodic table with its merits and limitations.
Ans:
Mendeleev’s Periodic Table:
Based on atomic mass
Elements arranged in groups and periods
Merits:
Left gaps for undiscovered elements
Predicted eka-boron, eka-aluminium, eka-silicon
Corrected atomic masses
Limitations:
No fixed position for hydrogen
No place for isotopes
Co–Ni anomaly
Q3. Explain trends in the modern periodic table.
Ans:
Valency: Same in a group, varies across a period
Atomic size: Decreases across a period, increases down a group
Metallic character: Decreases across a period, increases down a group
Non-metallic character: Increases across a period, decreases down a group
❓ FAQs (Concept Clarity – 5)
Q1. Why modern periodic table is better than Mendeleev’s table?
Ans: Because it is based on atomic number, not atomic mass.
Q2. Why isotopes have same position in modern periodic table?
Ans: Because isotopes have same atomic number.
Q3. Why atomic size increases down a group?
Ans: Due to addition of new shells.
Q4. Why metallic character decreases across a period?
Ans: Due to increase in nuclear charge.
Q5. Why elements in same group show similar properties?
Ans: Because they have same number of valence electrons.
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