Introduction to Classification of Elements

Classification of Elements is a fundamental chapter in NEET Chemistry. It helps students understand the organization of elements based on atomic structure and predicts chemical properties.

This chapter mainly focuses on modern periodic table, periodic trends, and formulas for atomic and ionic properties.

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♦ Some Basic Concepts of Chemistry ⇒ Download Now

♦ Structure of Atom ⇒ Download Now

♦ Classification of Elements and Periodicity in Properties ⇒ Download Now

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♦ s-Block Elements (Alkali and Alkaline Earth Metals) ⇒ Download Now

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♦ General Principles and Processes of Isolation of Elements ⇒ Download Now

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Importance of Classification for NEET

• Forms the foundation for understanding periodic trends and chemical reactivity

• Helps in solving numerical and conceptual questions

• Frequently asked in NEET for atomic and periodic properties

• Memorizing formulas speeds up calculation-based problems

Historical Classification of Elements

1. Dobereiner’s Triads

• Grouped elements in triads with similar properties

• Middle element atomic mass ≈ average of other two

Formula:
M₂ = (M₁ + M₃) / 2
Where M₂ = atomic mass of middle element

2. Newlands’ Law of Octaves

• Properties of every 8th element repeat

• Useful for early periodic patterns

3. Mendeleev’s Periodic Table

• Arranged elements by increasing atomic mass

• Left gaps for undiscovered elements

4. Modern Periodic Table (Moseley)

• Arranged by increasing atomic number (Z)

• Resolves anomalies in Mendeleev’s table

Modern Periodic Law

“Properties of elements are a periodic function of their atomic number (Z).”
This law is the foundation of periodicity in properties.

Periods and Groups

• Periods: Horizontal rows, properties change gradually

• Groups: Vertical columns, elements have similar valence electron configurations

NEET Tip: Learn group number, valence electrons, and representative elements.

Electronic Configuration and Periodicity

Formula/Concept:

• Valence electrons determine chemical properties

• General formulas:

  • s-block: ns¹–²

  • p-block: ns² np¹–⁶

  • d-block: (n–1)d¹–¹⁰ ns⁰–²

  • f-block: (n–2)f¹–¹⁴ (n–1)d⁰–¹ ns²

Electronic configuration explains periodic trends.

Atomic Radius (r)

Definition: Distance from nucleus to outermost electron

Trend Formulas:

• Across period: r decreases → r ∝ 1/Z_eff

• Down group: r increases → r ∝ n²/Z_eff

Approximate Formula:
r ≈ n² / Z_eff

Where n = principal quantum number, Z_eff = effective nuclear charge

Ionic Radius

• Cations: Smaller than parent atom (less electrons, more attraction)

• Anions: Larger than parent atom (more electrons, less attraction)

Trend:

• Across period: decreases for cations and anions

• Down group: increases

Ionization Energy (IE)

Definition: Energy required to remove 1 mole of electrons from 1 mole of atoms in gaseous state

Formula:
IE ∝ Z_eff² / n²

Trend:

• Across period: increases

• Down group: decreases

Successive Ionization Energy:
IE₂ > IE₁ > IE₃ …

Electron Affinity (EA)

Definition: Energy change when an atom gains an electron

Trend:

• Across period: generally increases

• Down group: decreases

Formula Concept:
EA ∝ Z_eff / n²

Electronegativity (χ)

Definition: Ability of an atom to attract electrons in a bond

Pauling Scale: 0–4

Trend Formulas:

• Across period: increases

• Down group: decreases

Approximate Calculation:
χ ∝ IE + EA

Metallic and Non-Metallic Character

• Metals: Lose electrons → metallic character decreases across period, increases down group

• Non-metals: Gain electrons → non-metallic character increases across period, decreases down group

NEET Tip: Compare atomic radius, IE, and electronegativity to predict reactivity.

Valency of Elements

Definition: Number of electrons an atom can gain, lose, or share

Formulas:

• Main group elements: Valency = Number of valence electrons (s and p)

• Oxidation states vary for transition elements

Example:

• Group 1: Valency = 1

• Group 17: Valency = 1 (for forming ionic compounds)

Periodic Trends Summary Table (NEET)

Importance of Periodicity in NEET

• Explains reactivity patterns of elementsHelps predict physical and chemical properties

• Foundation for organic and inorganic chemistry chapters

• Frequently asked in MCQs, assertion-reasoning, and numerical questions

Conclusion

Classification of Elements and Periodicity in Properties is a high-scoring chapter for NEET Chemistry. Mastering formulas related to atomic radius, ionization energy, electron affinity, and electronegativity is crucial.

• Helps in solving numerical and conceptual questions

• Builds strong foundation for chemical reactivity and bonding

• Regular practice ensures speed and accuracy in exams

For more NEET Chemistry formulas, notes, and preparation material, visit StudentBro.in, your trusted platform for exam success.