Valency

Comprehensive Note on Valency & Bonding

Tags: #chemistry #valency #chemical-bonding #board-prep #jee-neet #olympiad

1. The Concept of Valency (যোজ্যতা)

At its core, Valency (যোজ্যতা) is the combining capacity of an element. It dictates how many chemical bonds an atom of a given element can form with other atoms to achieve a stable electronic configuration.

• Valence Shell (যোজ্যতা কক্ষ): The outermost electron shell of an atom.

• Valence Electrons (যোজ্যতা ইলেকট্রন): The electrons present in the outermost shell. These are the only electrons that typically participate in chemical bonding.

• Formula: $|\text{Periodic Number of the Atom}-\text{Periodic Number of the Nearest Noble Gas}|$

The Driving Force: Octet and Duplet Rules

Atoms combine to achieve the highly stable, low-energy electron configuration of their nearest Noble Gas (নিষ্ক্রিয় গ্যাস).

• Octet Rule (অষ্টক সূত্র): Atoms tend to gain, lose, or share electrons until they are surrounded by eight valence electrons.

• Duplet Rule (দ্বৈত সূত্র): For elements close to Helium (like H, Li, Be), stability is achieved with exactly two electrons in their single outer shell (the K shell).

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2. Types of Valency (যোজ্যতার প্রকারভেদ)

Depending on how an atom achieves its octet, valency is classified into three main types:

A. Electrovalency / Ionic Bonding (তড়িৎযোজ্যতা বা আয়নীয় বন্ধন)

Occurs when one or more electrons are completely transferred from a metal to a non-metal.

• The number of electrons lost or gained by an atom is its electrovalency.

• Example: In NaCl, Sodium (Na) loses 1 electron (Electrovalency = 1) to become $N{a}^{+}$, and Chlorine (Cl) gains 1 electron (Electrovalency = 1) to become $C{l}^{-}$.

B. Covalency / Covalent Bonding (সমযোজ্যতা)

Occurs when atoms mutually share pairs of electrons to achieve an octet. Usually happens between non-metals.

• The number of electron pairs an atom shares is its covalency.

• Example: In $H_{2}O$, Oxygen shares 2 of its electrons (one with each Hydrogen), so the covalency of Oxygen is 2.

C. Coordinate Covalency / Dative Bonding (অসমযোজ্যতা)

A special type of covalent bond where the shared pair of electrons is provided by only one of the bonding atoms.

• The atom providing the electron pair is the Donor (দাতা), and the atom accepting it is the Acceptor (গ্রহীতা).

• Example: In the Ammonium ion ($N{H}_4^{+}$), the Nitrogen atom of $N{H}_3$ donates its lone pair of electrons to an empty orbital of an $H^{+}$ ion.

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3. General Rules for Determining Valency (যোজ্যতা নির্ণয়ের সাধারণ নিয়ম)

For representative elements (s-block and p-block), valency can usually be predicted by their Group number in the periodic table.

1. Groups 1, 2, 13, and 14: * Valency = Number of valence electrons.

   • Example: Carbon (Group 14) has 4 valence electrons $\to$ Valency = 4.

2. Groups 15, 16, and 17:

   • Valency = 8 - (Number of valence electrons).

   • Example: Nitrogen (Group 15) has 5 valence electrons $\to$ Valency = $8-5=3$.

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4. Variable Valency (পরিবর্তনশীল যোজ্যতা)

Some elements exhibit more than one valency in different compounds. This is primarily seen in:

• Transition Metals (সন্ধিগত মৌল - d-block): The energy difference between the outermost ‘s’ orbital and the penultimate ‘d’ orbital is very small. Therefore, electrons from both shells can participate in bonding under varying thermodynamic conditions.

  • Example - Iron (Fe): Can show a valency of 2 (Ferrous / $F{e}^{2+}$) or 3 (Ferric / $F{e}^{3+}$).

  • Example - Copper (Cu): Can show a valency of 1 (Cuprous / $C{u}^{+}$) or 2 (Cupric / $C{u}^{2+}$).

• Heavier p-block Elements (Inert Pair Effect / নিষ্ক্রিয় জোড় প্রভাব): Elements like Tin (Sn) and Lead (Pb) show variable valencies (e.g., Pb shows +2 and +4). The $+2$ state becomes highly stable down the group because the deeply buried s-electrons strongly resist participating in bonding.

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5. Advanced / Competitive Concepts (উচ্চতর ধারণা)

Valency vs. Oxidation State (যোজ্যতা বনাম জারণ সংখ্যা)

This is a critical distinction for advanced chemistry problems. They are related but mathematically and conceptually distinct.

• Valency: A pure number representing combining capacity. It never has a sign (+ or -) and cannot be zero (except for noble gases) or a fraction.

• Oxidation State (জারণ সংখ্যা): The hypothetical electrical charge an atom would have if all bonds were 100% ionic. It must have a sign (+ or -), and can be zero or even fractional in complex molecules.

  • Example in $C{H}_4$: Carbon’s valency is 4. Its oxidation state is -4.

  • Example in $C{O}_2$: Carbon’s valency is 4. Its oxidation state is +4.

Hypervalency / The Expanded Octet (সম্প্রসারিত অষ্টক)

The Octet Rule is not universal. Elements from the 3rd period onwards (like P, S, Cl) can accommodate more than 8 electrons in their valence shell.

• Cause: The availability of empty, low-lying d-orbitals that can accept electrons.

• Example ($PC{l}_5$): Phosphorus forms 5 bonds, having 10 valence electrons around it.

• Example ($S{F}_6$): Sulfur forms 6 bonds, having 12 valence electrons around it.