Class 10th || Science || Notes || Chapter 5: Periodic Classification of Elements

Chapter 5: Periodic Classification of Elements

The chemical world is vast, with over 118 known elements. Studying their properties one by one is tedious and impractical. To bring order to this complexity, scientists developed methods to classify elements systematically. This chapter explores how elements are classified, their arrangement in the periodic table, and the trends that emerge.

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1. Need for Classification

As the number of known elements increased, scientists faced challenges in understanding and studying their properties. Classification became necessary to:

1. Organize elements based on their properties.
2. Identify trends and relationships among elements.
3. Predict the behavior and properties of elements, including undiscovered ones.

Imagine a library without proper arrangement of books; finding one would be chaotic. Similarly, classifying elements into groups and periods makes studying chemistry systematic and efficient.

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2. Early Attempts at Classification

(a) Dobereiner’s Triads (1817)

Johann Wolfgang Dobereiner identified groups of three elements, called triads, that had similar properties. The atomic mass of the middle element in each triad was approximately the average of the other two.

Example:

Element	Atomic Mass
Lithium	7
Sodium	23
Potassium	39

Here, $(7 + 39)/2 = 23$.

Limitations:

1. Only a few triads were identified.
2. This method failed when more elements were discovered.

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(b) Newlands’ Law of Octaves (1864)

John Newlands arranged elements in increasing order of atomic mass. He observed that every eighth element showed similar properties, just like the eighth note in a musical scale.

Example:

• Lithium and Sodium, the first and eighth elements in his arrangement, have similar properties.

Limitations:

1. This pattern worked only for elements up to calcium.
2. Properties of heavier elements did not follow the law of octaves.
3. Newlands forced some elements into his arrangement, even when their properties didn’t match.

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3. Mendeleev’s Periodic Table (1869)

Dmitri Mendeleev made the first significant breakthrough in organizing elements. He arranged them in increasing order of atomic mass and grouped them based on their chemical properties.

Key Features of Mendeleev’s Table:

1. Groups and Periods:

   • Vertical columns called groups contained elements with similar properties.
   • Horizontal rows called periods showed gradual changes in properties.

2. Gaps for Undiscovered Elements:

   • Mendeleev left gaps in his table for elements that were yet to be discovered.
   • He accurately predicted the properties of these elements, such as gallium and germanium, which were later discovered and matched his predictions.

3. Periodic Law:

   • "The properties of elements are a periodic function of their atomic masses."

Strengths:

• Mendeleev’s table was widely accepted because it helped predict the properties of undiscovered elements.
• It grouped elements with similar properties together.

Limitations:

1. Isotopes, which have different atomic masses but similar properties, could not be explained.
2. The position of hydrogen was uncertain.
3. Arranging elements strictly by atomic mass led to anomalies (e.g., iodine and tellurium).

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4. Modern Periodic Table

The limitations of Mendeleev’s table were resolved with the discovery of the atomic number (number of protons in an atom). The modern periodic table is based on the atomic number rather than atomic mass.

Modern Periodic Law:

"The properties of elements are a periodic function of their atomic numbers."

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5. Structure of the Modern Periodic Table

1. Groups (Vertical Columns):

   • The periodic table has 18 groups.
   • Elements in a group have the same number of valence electrons, which gives them similar chemical properties.
   • Example: Group 1 elements (alkali metals) like sodium, potassium, and lithium all have 1 valence electron.

2. Periods (Horizontal Rows):

   • The table has 7 periods, each corresponding to the number of shells in the atom.
   • As we move across a period, the number of valence electrons increases, while the number of shells remains the same.

3. Classification of Elements:

   • Metals: Found on the left and center of the table (e.g., sodium, magnesium, iron).
   • Non-metals: Found on the right side (e.g., oxygen, chlorine).
   • Metalloids: Found along the zig-zag line (e.g., silicon, boron), exhibiting properties of both metals and non-metals.

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6. Periodic Trends in Properties

(a) Atomic Radius:

• The distance between the nucleus and the outermost shell of an atom.
• Trend:
  • Across a Period: Atomic radius decreases due to increased nuclear charge pulling electrons closer.
  • Down a Group: Atomic radius increases because of additional shells.

(b) Valency:

• The number of electrons an atom needs to gain, lose, or share to achieve a stable configuration.
• Trend:
  • Across a period: Increases from 1 to 4, then decreases to 0.
  • Down a group: Remains the same.

(c) Metallic and Non-metallic Character:

• Metallic Character: Tendency to lose electrons.
  • Trend: Decreases across a period, increases down a group.
• Non-metallic Character: Tendency to gain electrons.
  • Trend: Increases across a period, decreases down a group.

(d) Ionization Energy:

• The energy required to remove an electron from an atom.
• Trend:
  • Increases across a period (harder to remove electrons).
  • Decreases down a group (easier to remove electrons due to larger atomic size).

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7. Advantages of the Modern Periodic Table

1. Elements are arranged logically based on their atomic structure.
2. It explains periodic trends and predicts properties of elements.
3. It helps in understanding chemical behavior systematically.
4. The table provides a framework for studying compounds formed by elements.

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8. Applications of the Periodic Table

• Understanding Reactivity: Group 1 elements (alkali metals) are highly reactive, while noble gases (Group 18) are inert.
• Predicting Compounds: Knowing the valency of elements helps predict the compounds they form.
• Material Development: Scientists use periodic trends to discover new materials for industrial applications.

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9. Key Terms and Definitions

1. Periodic Law: The repetition of properties at regular intervals when elements are arranged in increasing atomic number.
2. Group: Vertical columns in the periodic table with elements having similar properties.
3. Period: Horizontal rows in the periodic table with elements showing gradual changes in properties.
4. Metalloids: Elements that show properties of both metals and non-metals, e.g., silicon.

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10. Suggested Activities for Students

1. Create a 3D model of the periodic table.
2. Research and present the uses of any two elements from different groups.
3. Observe and plot trends like atomic radius and metallic character using data from the periodic table.

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Key Learning Outcomes

• Recognize the importance of periodic classification in understanding the properties of elements.
• Learn trends like atomic size, valency, metallic character, and ionization energy.
• Appreciate the predictive power of the periodic table in chemistry.
• Understand how the periodic table is a unifying framework for the study of elements.

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