CBSE Notes Class 10 Science - Periodic Classification of Elements
Understanding the organization of elements is fundamental in chemistry. Chapter 5 of the CBSE Class 10 Science syllabus, "Periodic Classification of Elements," provides insight into how elements are categorized based on their properties
1.0Early Attempts of Classification
- Dobereiner’s Triads:
Johann Wolfgang Döbereiner, a German chemist, arranged elements into groups of three, known as triads.
According to this, the atomic mass of the middle element was approximately the average of the other two elements. However, this method was limited to only a few elements.
Examples-
- Newlands’ Law of Octaves:
John Newlands, an English chemist, suggested that elements show similar properties every eighth element, similar to musical octaves.
Here is the table above showcases a unique correlation between the Indian musical notes "sa re ga ma pa da ni" and their Western counterparts "do re mi fa so la ti," paired with elements from the periodic table.
Limitations of Newlands’ Law of Octaves
- Applicable only up to calcium; every eighth element beyond did not show similar properties.
- Assumed only 56 elements existed and no new ones would be discovered.
- Placed dissimilar elements like cobalt and nickel with fluorine, chlorine, and bromine.
- Iron, which resembles cobalt and nickel, was placed far away from them.
- Worked well only for lighter elements; failed for heavier ones.
- Mendeleev’s Periodic Table:
Dmitri Mendeleev's table organized elements in increasing order of atomic masses and grouped them by their chemical properties.
He left gaps for undiscovered elements and even corrected the atomic masses of some elements. This table laid the foundation for the modern periodic table but had limitations like incorrect placement of certain elements due to atomic mass anomalies. Here is a brief description of the achievements and limitations of Mendeleev’s Periodic Table:
Achievements:
- Organized elements by atomic mass and properties.
- Predicted properties of undiscovered elements.
- Corrected atomic masses of some elements.
- Laid the foundation for the modern periodic table.
Limitations:
- Misplaced some elements due to atomic mass anomalies.
- Could not accommodate isotopes.
- Unclear placement of hydrogen.
- Excluded noble gases, as they were not discovered yet.
2.0Modern Periodic Table
The modern periodic table, based on increasing atomic numbers, resolved Mendeleev’s inconsistencies. Elements are arranged in rows called periods and columns called groups. Elements in the same group have similar chemical properties because they have the same number of valence electrons.
3.0Features of the Modern Periodic Table
Groups and Periods:
- Groups: There are 18 groups in the periodic table. Elements in a group have similar properties.
- Periods: There are 7 periods. The number of elements in each period varies, reflecting the filling of electron shells.
Trends in the Periodic Table:
- Atomic Size: Decreases from left to right across a period due to increased nuclear charge. Increases down a group as new electron shells are added.
- Ionization Energy: Increases across a period and decreases down a group.
- Electronegativity: Generally increases across a period and decreases down a group.
- Metallic and Non-metallic Character: Metallic character decreases across a period and increases down a group, while non-metallic character shows the opposite trend.
4.0Importance of Periodic Classification
Predicting Properties: Understanding periodic trends helps predict the behavior of elements in chemical reactions.
Grouping Similar Elements: Makes studying elements easier by grouping those with similar properties.
Identifying Elemental Trends: Helps in identifying trends like reactivity, melting points, and more.
5.0Important Concepts of Periodic Table
- Valency: The combining capacity of an element. It relates to the number of electrons an element can donate, accept, or share.
- Atomic and Ionic Size: Important for understanding reactivity and bonding.
- Metallic and Non-metallic Properties: Helps in predicting the type of bonding and reactivity of elements.
