CBSE Class 10 Science Notes Chapter 5 Periodic Classification of Elements

Periodic classification of elements class 10 notes offer a structured summary of how scientists organized the building blocks of matter. These notes cover the historical evolution from Dobereiner’s Triads and Newlands’ Octaves to Mendeleev’s Periodic Table, culminating in the Modern Periodic Table, which organizes 118 known elements based on their atomic numbers and recurring chemical properties.

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Periodic Classification of Elements Class 10 Notes

To start with, very few elements were known in the initial stages of chemistry. As more elements were detected, there arose a need for them to be classified. By following the periodic classification of elements class 10 notes, one can easily identify the rationale behind the classification of elements by earlier chemists. Students always seek a periodic classification of elements class 10 notes pdf for easy accessibility of these milestones.

Dobereiner’s Triads

The beginning of the classification was initiated by Johann Wolfgang Dobereiner. Elements were categorized in groups of three. These were called "triads." The rule was as follows: when the elements are listed in increasing order of their increasing atomic masses, the average of the other two atomic masses would approximate the other element.

Example of a Dobereiner Triad:

• Lithium (Li): 6.9

• Sodium (Na): 23.0

• Potassium (K): 39.0 Average mass calculation: (6.9 + 39.0) / 2 = 22.95 (very close to 23).

Newlands’ Law of Octaves

John Newlands noticed a pattern similar to musical notes. He arranged elements by increasing atomic mass and found that every eighth element shared properties with the first. However, this law only worked well up to Calcium and assumed only 56 elements existed in nature.  

Mendeleev’s Periodic Table

Dmitri Mendeleev is often called the father of the periodic table. His law stated: "The properties of elements are a periodic function of their atomic masses."  

• Achievements: He left gaps for undiscovered elements (like Eka-boron and Eka-aluminium) and predicted their properties accurately.  

• Limitations: He couldn't give a fixed position to Hydrogen, and isotopes posed a problem since they have different masses but the same chemical properties.

Whether you are studying for a periodic classification of elements class 10 notes state board exam or looking for a periodic classification of elements class 10 notes pdf download, understanding Mendeleev’s work is essential as it paved the way for modern chemistry.

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The Modern Periodic Table: Trends and Characteristics

The Modern Periodic Table shifted the focus from atomic mass to atomic number. Henry Moseley demonstrated that atomic number is a more fundamental property. The Modern Periodic Law states: "Properties of elements are a periodic function of their atomic number."  

Structure of the Table

The table is organized into horizontal rows called Periods (7 in total) and vertical columns called Groups (18 in total). Elements in the same group have the same number of valence electrons, which is why they react in similar ways.  

Trends in the Modern Periodic Table

If you are following periodic classification of elements class 10 notes prashant kirad or other popular study guides, you will notice a heavy emphasis on these specific trends:

1. Valency Valency is determined by the number of valence electrons in the outermost shell.  

• Across a period: Valency first increases from 1 to 4 and then decreases to 0.

• Down a group: Valency remains the same because the number of valence electrons is constant.  

2. Atomic Size This refers to the distance between the center of the nucleus and the outermost shell.

• Across a period: Atomic radius decreases. This is because the nuclear charge increases, pulling the electrons closer to the nucleus.

• Down a group: Atomic size increases. New shells are added, which increases the distance between the nucleus and the outer electrons, outweighing the increased nuclear charge.

3. Metallic and Non-Metallic Properties

• Metallic Character: Metals tend to lose electrons (electropositive). Metallic character decreases across a period and increases down a group.  

• Non-Metallic Character: Non-metals tend to gain electrons (electronegative). This character increases across a period and decreases down a group.  

• Metalloids: Elements like Boron, Silicon, and Germanium show properties of both metals and non-metals and are found along a zig-zag line in the table.  

4. Electronegativity This is the tendency of an atom to attract a shared pair of electrons.

• Across a period: Electronegativity increases due to higher nuclear charge.  

• Down a group: Electronegativity decreases as the atomic size increases. 

Read More: NCERT Solutions for Class 10 Science Chapter 5

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CBSE Class 10 Science Notes Chapter 5 FAQs

Why did Mendeleev leave gaps in his periodic table?

Mendeleev was bold enough to realize that not all elements had been discovered. He left gaps for these missing elements and even predicted their properties based on their position. When elements like Gallium and Germanium were later found, they matched his predictions perfectly.

What is the fundamental difference between Mendeleev’s and the Modern Periodic Table?

Mendeleev arranged elements based on their atomic mass. The Modern Periodic Table, however, uses the atomic number (the number of protons in the nucleus). Using atomic numbers resolved several issues, such as the placement of isotopes and the irregular increase in atomic masses.  

How does atomic size change as we move from left to right in a period?

Atomic size decreases as you move left to right across a period. Even though the number of electrons increases, they are added to the same shell. Meanwhile, the number of protons in the nucleus increases, creating a stronger positive pull that draws the electrons inward.  

What are metalloids or semi-metals?

Metalloids are elements that act as a bridge between metals and non-metals. They possess characteristics of both. In the periodic table, they are located along a zig-zag line that separates the highly reactive metals on the left from the non-metals on the right.  

Why do elements in the same group show similar chemical properties?

Chemical properties are determined by valence electrons. Since all elements in a specific group have the same number of electrons in their outermost shell, they react similarly with other substances to achieve a stable configuration.