Valency

Valency is defined as the combining capacity of an element. It dictates how atoms of an element interact, bond, and combine with atoms of other elements to form chemical compounds.

Valence Electrons vs. Valency

It is critical to distinguish between valence electrons and valency, as they are related but distinct concepts:

• Valence Electrons: The total number of electrons present in the outermost shell (valence shell) of an atom.
• Valency: The actual number of electrons an atom needs to lose, gain, or share to attain a stable, completely filled outer shell.

The Octet Rule and Chemical Stability

Proposed by Gilbert N. Lewis and Albrecht Kossel, the Octet Rule states that atoms are most stable when their outermost valence shell contains exactly eight electrons.

• The Exception: For the innermost shell (K-shell), stability is achieved with just two electrons (known as a Duplet), as seen in Helium (He).
• The Driving Force of Reactions: Atoms with fewer than eight electrons in their outer shell are chemically reactive. They undergo chemical combinations—via ionic or covalent bonding—to complete their octet or duplet configuration.

Calculation Matrix for Valency

The valency of an element is directly derived from its valence electrons. The calculation follows two simple rules depending on the population of the outer shell.

Rule 1: For Valence Electrons 1 to 4

If the number of electrons in the outermost shell is 4 or less, the valency is equal to the number of valence electrons. These atoms generally prefer to lose or share electrons.

Rule 2: For Valence Electrons 5 to 8

If the number of electrons in the outermost shell is 5 or more, the valency is determined by subtracting the number of valence electrons from 8. These atoms generally prefer to gain or share electrons to complete their octet.

Analytical Reference Table (Elements 1 to 20)

Classification of Elements Based on Valency

Elements are often grouped based on their combining capacity, which determines the chemical formulas of the compounds they form.

• Monovalent (Valency = 1): Atoms that exchange or share a single electron. Examples include Hydrogen (H), Sodium (Na^+), Potassium (K^+), and Chlorine (Cl^-).
• Divalent (Valency = 2): Atoms that exchange or share two electrons. Examples include Magnesium (Mg²⁺), Calcium (Ca²⁺), and Oxygen (O^2-).
• Trivalent (Valency = 3): Atoms that exchange or share three electrons. Examples include Aluminium (Al³⁺) and Nitrogen (N^3-).
• Tetravalent (Valency = 4): Atoms that share four electrons. Carbon (C) and Silicon (Si) are classic examples, forming the foundation of organic chemistry and semiconductor technology respectively.

Variable Valency

Certain elements, particularly transition metals (like Iron, Copper, and Manganese) and some non-metals (like Phosphorus and Sulfur), exhibit more than one valency depending on the nature of the chemical reaction and experimental conditions. This occurs because these elements can lose electrons not only from their outermost shell but also from the subshell just below it (d-orbitals).

• Iron (Fe): Exhibits a valency of 2 in Ferrous compounds (Fe²⁺, e.g., FeSO₄) and a valency of 3 in Ferric compounds (Fe³⁺, e.g., FeCl₃).
• Copper (Cu): Exhibits a valency of 1 in Cuprous compounds (Cu^+, e.g., Cu₂O) and a valency of 2 in Cupric compounds (Cu²⁺, e.g., CuO).

Application of Valency: Writing Chemical Formulas

Valency is used to derive the empirical and molecular formulas of compounds using the Criss-Cross Method. In this method, the valencies of the combining atoms or radicals are crossed over to become the subscripts of the opposing element.

Step-by-Step Examples

• Magnesium Chloride Formula:
  • Write the constituent symbols side-by-side: Mg and Cl.
  • Write their respective valencies below them: Mg = 2, Cl = 1.
  • Cross over the valencies. The 1 goes to Mg and the 2 goes to Cl.
  • Resulting Formula: MgCl₂
• Aluminium Oxide Formula:
  • Constituent symbols: Al and O.
  • Respective valencies: Al = 3, O = 2.
  • Cross over the valencies.
  • Resulting Formula: Al₂O₃

Core Trivia for Civil Services Examination

• Zero Valency Element Group: The elements of Group 18 of the modern periodic table (Helium, Neon, Argon, Krypton, Xenon, and Radon) are known as Noble Gases or Inert Gases. Because their outermost shells are completely filled, their valency is zero, making them highly unreactive under standard conditions.
• The Semiconductor Backbone: Carbon and Silicon both have a valency of 4. However, while Carbon forms strong covalent chains enabling life (catenation), Silicon’s stable tetravalent crystal structure allows it to be doped easily with trivalent (e.g., Boron) or pentavalent (e.g., Phosphorus) impurities. This makes Silicon the foundational element for modern semiconductor chips and solar photovoltaic cells.
• Polyatomic Ion Valency: Valency also applies to clusters of atoms that carry a net charge, known as polyatomic ions. For instance, the Ammonium radical (NH₄^+) has a valency of 1, the Carbonate radical (CO₃^2-) has a valency of 2, and the Phosphate radical (PO₄^3-) has a valency of 3. These groups cross over their net charge as a single unit during chemical bonding.