Periodic Classification Of elements Notes

📘 Class 10 Science – Periodic Classification of Elements Notes

🔷 Introduction

Classification helps in the systematic study of elements.
The number of known elements increased rapidly (now > 118), making it necessary to organize them in a proper way.
Earlier attempts were made before the modern Periodic Table.

🔷 Dobereiner’s Triads (1817)

Grouped elements in sets of three with similar properties.
The atomic mass of the middle element was approximately the mean of the other two.

Example:

Element	Atomic Mass
Lithium	7
Sodium	23
Potassium	39

Average of Li and K = (7 + 39)/2 = 23
So, Sodium fits well.

🔹 Limitation: Could not be applied to all known elements.

🔷 Newlands’ Law of Octaves (1866)

When elements are arranged in increasing order of atomic mass, every 8th element showed properties similar to the first (like octaves in music).

Example:
Na (11), K (19), Rb (37) – show similar properties.

🔹 Limitation:

Valid only up to Calcium.
Did not work with heavier elements.
Grouping of dissimilar elements.

🔷 Mendeleev’s Periodic Table (1869)

Arranged elements in increasing atomic mass.
Elements with similar properties were placed in the same column (group).

Achievements:

Left gaps for undiscovered elements and predicted their properties.
Adjusted positions of some elements to fit properties.

Example:
He named an element “Eka-aluminium” (discovered later as Gallium).

Element	Atomic Mass	Group
Gallium (Ga)	69.7	III A
Eka-Aluminium	~68	III A

Limitations:

No fixed position for Hydrogen.
No explanation for isotopes.
Position of some elements disturbed the order.

🔷 Modern Periodic Table (Henry Moseley, 1913)

Arranged elements in increasing order of atomic number (Z).
Solved many anomalies of Mendeleev’s table.

🔷 Structure of the Modern Periodic Table

Feature	Description
Total Periods	7 (horizontal rows)
Total Groups	18 (vertical columns)
First Period	Contains only 2 elements
Last Period	Contains many elements (Lanthanides & Actinides)

🔷 Trends in the Modern Periodic Table

1. Valency

Valency = No. of electrons gained/lost/shared to complete octet.
Across a period: First increases, then decreases.
Down a group: Remains the same.

Example:
Group 1 (Na, K) – Valency = 1

2. Atomic Size

Distance from nucleus to outermost shell.

Trend	Explanation
Across a period	Decreases (more nuclear pull)
Down a group	Increases (new shell added)

Example:
Li > Be > B > C (size decreases across)

3. Metallic & Non-metallic Character

Metallic character: Tendency to lose electrons.
Non-metallic character: Tendency to gain electrons.

Trend	Metallic	Non-metallic
Across Period	Decreases	Increases
Down Group	Increases	Decreases

Example:
Na is more metallic than Mg.
O is more non-metallic than N.

4. Electronegativity

Ability to attract shared electrons.

5. Ionization Energy

Energy required to remove an electron.

🔷 Position of Elements

Element	Period	Group	Type
Hydrogen	1	1	Non-metal
Sodium	3	1	Metal
Chlorine	3	17	Non-metal
Argon	3	18	Noble Gas

🔷 Metalloids

Show both metallic and non-metallic properties.
Example: Boron, Silicon, Arsenic

🔷 Advantages of Modern Table

Clear grouping and trends.
Helped predict chemical behavior easily.
Based on atomic number (a better property than mass).

Let me know if you’d like diagrams like the Periodic Table or the position of elements in groups/periods!