Quick Revision Notes

Class 10 Science - Periodic Classification of Elements

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1. Introduction & Need for Classification

• Classification: The arrangement of elements with similar properties into groups to make the study of a large number of elements easier and systematic.
• Early attempts: Elements were initially classified into Metals and Non-Metals.

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2. Early Models of Classification

A. Dobereiner’s Triads (डobereiner के त्रिक)

• Statement: When elements are arranged in order of increasing atomic masses, groups of three elements (known as triads) having similar chemical properties are formed. The atomic mass of the middle element is roughly the arithmetic mean of the atomic masses of the other two elements.
• Example: Sodium (Na) is the mean of Lithium (Li) and Potassium (K).
  • Atomic mass of Li = 6.9, K = 39.1
  • Mean = (6.9 + 39.1) / 2 = 23 (Atomic mass of Na)
• Limitation: Could only identify a few triads; failed for heavier elements.

B. Newlands’ Law of Octaves (न्यूलैंड्स का अष्टक नियम)

• Statement: When elements are arranged in order of increasing atomic masses, the properties of every eighth element are a repetition of the properties of the first element (similar to musical notes: Sa, Re, Ga, Ma, Pa, Dha, Ni).
• Limitation:
  • Applicable only up to Calcium (Ca).
  • Assumed only 56 elements existed in nature and no more would be discovered.
  • Placed two elements in the same slot (e.g., Co and Ni).

C. Mendeleev’s Periodic Table (मैंडलीफ की आवर्त सारणी)

• Basis: Arranged elements in increasing order of their Atomic Masses and similarities in chemical properties (specifically hydrides and oxides).
• Mendeleev’s Periodic Law: The properties of elements are a periodic function of their atomic masses.
• Features:
  • Contains vertical columns called Groups (8 groups) and horizontal rows called Periods (7 periods).
  • Left gaps for undiscovered elements (e.g., Eka-boron, Eka-aluminium, Eka-silicon).
• Merits:
  • Systematic study of elements.
  • Predicted properties of yet-to-be-discovered elements (Scandium, Gallium, Germanium).
• Demerits:
  • Position of isotopes could not be explained.
  • Anomaly in atomic masses (e.g., Cobalt with higher atomic mass was placed before Nickel).
  • Position of Hydrogen was uncertain.

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3. Modern Periodic Table (आधुनिक आवर्त सारणी)

• Henry Moseley (1913): Showed that atomic number is a more fundamental property than atomic mass.
• Modern Periodic Law: The properties of elements are a periodic function of their atomic numbers (Z).
• Structure:
  • Periods (आवर्त): 7 horizontal rows.
  • Groups (समूह): 18 vertical columns. Elements in the same group have the same number of valence electrons and similar chemical properties.

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4. Trends in the Modern Periodic Table (आवर्त सारणी में प्रवृत्तियाँ)

A. Valency (संयोजकता)

• Definition: Combining capacity of an atom, equal to the number of valence electrons or 8 minus the valence electrons.
• Across a Period (Left to Right): First increases from 1 to 4, then decreases to 0.
• Down a Group (Top to Bottom): Remains the same (valency is equal for all elements in a group).

B. Atomic Size / Atomic Radius (परमाणु आकार / त्रिज्या)

• Definition: Distance from the center of the nucleus to the outermost shell of an isolated atom.
• Across a Period (Left to Right): Decreases. (Reason: Nuclear charge increases, pulling electrons closer to the nucleus).
• Down a Group (Top to Bottom): Increases. (Reason: New shells are added, increasing the distance between nucleus and outermost shell).

C. Metallic and Non-Metallic Properties (धात्विक और अधात्विक गुण)

• Metallic Character (Tendency to lose electrons):
  • Across a Period: Decreases (Left to right elements become less metallic / more non-metallic).
  • Down a Group: Increases (Elements lose electrons easily due to larger atomic size).
• Non-Metallic Character (Tendency to gain electrons):
  • Across a Period: Increases.
  • Down a Group: Decreases.
• Note: Metals are on the left/center, Non-metals on the right, and Metalloids (उपधातु) like Silicon, Boron, Germanium lie along the zig-zag line separating metals and non-metals.

D. Electronegativity (विद्युत ऋणात्मकता)

• Ability to attract shared pair of electrons.
• Across a Period: Increases.
• Down a Group: Decreases.

E. Nature of Oxides (ऑक्सीड की प्रकृति)

• Basic Oxides: Formed by metals (e.g., $Na_2O$, $MgO$).
• Acidic Oxides: Formed by non-metals (e.g., $CO_2$, $SO_2$).
• Amphoteric Oxides: Show both acidic and basic behavior (e.g., $Al_2O_3$, $ZnO$).

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Quick Memory Tips for Exam

1. Mendeleev = Atomic Mass | Moseley = Atomic Number
2. Left to Right (Period): Atomic size decreases, Metallic character decreases, Non-metallic character increases.
3. Top to Bottom (Group): Atomic size increases, Metallic character increases, Valency remains constant.