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Chemical Bonding - Lewis Approach-advanced

Grade 9CBSE

Review the key concepts, formulae, and examples before starting your quiz.

🔑Concepts

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The Lewis approach to chemical bonding is based on the concept of the octet rule, where atoms gain, lose, or share electrons to achieve a stable configuration of eight electrons in their valence shell (ns2np6ns^2 np^6).

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Lewis Symbols (Electron Dot Structures) represent valence electrons as dots around the chemical symbol of the element. For example, Lithium is represented as Li⋅Li\cdot and Magnesium as ⋅Mg⋅\cdot Mg \cdot.

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An Ionic Bond (Electrovalent Bond) is formed by the complete transfer of one or more electrons from one atom to another, resulting in the formation of cations and anions held together by electrostatic forces.

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A Covalent Bond is formed by the mutual sharing of one or more pairs of electrons between atoms. A single bond involves one pair (22 electrons), a double bond involves two pairs (44 electrons), and a triple bond involves three pairs (66 electrons).

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Formal Charge (FCFC) is the difference between the number of valence electrons in an isolated atom and the number of electrons assigned to that atom in a Lewis structure. It helps in identifying the most stable Lewis structure.

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Exceptions to the Octet Rule include the incomplete octet (e.g., LiClLiCl, BeH2BeH_2, BCl3BCl_3), odd-electron molecules (e.g., NONO, NO2NO_2), and the expanded octet where elements in the 3rd3^{rd} period and beyond can accommodate more than 88 electrons (e.g., PCl5PCl_5, SF6SF_6, H2SO4H_2SO_4).

📐Formulae

FC=[V]−[L]−12[S]FC = [V] - [L] - \frac{1}{2}[S]

Total Valence Electrons (Q)=∑Valence electrons of all atoms+(Negative charge)−(Positive charge)\text{Total Valence Electrons (Q)} = \sum \text{Valence electrons of all atoms} + (\text{Negative charge}) - (\text{Positive charge})

Bond Order=Total number of bonds between two atomsTotal number of resonating structures\text{Bond Order} = \frac{\text{Total number of bonds between two atoms}}{\text{Total number of resonating structures}}

💡Examples

Problem 1:

Calculate the Formal Charge of the central oxygen atom in the ozone (O3O_3) molecule.

Solution:

  1. The total number of valence electrons for O3O_3 is 3×6=183 \times 6 = 18.
  2. The Lewis structure of O3O_3 can be drawn as O(1)=O(2)−O(3)O(1)=O(2)-O(3).
  3. For the central atom (O(2)O(2)):
    • Valence electrons (VV) = 66
    • Lone pair electrons (LL) = 22 (one lone pair)
    • Shared electrons (SS) = 66 (three bonds)
  4. Using the formula: FC=6−2−12(6)FC = 6 - 2 - \frac{1}{2}(6) FC=6−2−3=+1FC = 6 - 2 - 3 = +1

Explanation:

The central oxygen atom in ozone has a formal charge of +1+1, indicating it has effectively 'lost' one electron density relative to its neutral state to maintain the bonding structure.

Problem 2:

Represent the formation of Magnesium Chloride (MgCl2MgCl_2) using Lewis electron dot structures.

Solution:

Magnesium (MgMg) has the configuration [Ne]3s2[Ne] 3s^2 (2 valence electrons). Chlorine (ClCl) has [Ne]3s23p5[Ne] 3s^2 3p^5 (7 valence electrons).

Step 1: MgMg loses 22 electrons to form Mg2+Mg^{2+}. Mg→Mg2++2e−Mg \rightarrow Mg^{2+} + 2e^-

Step 2: Two ClCl atoms each accept 11 electron to form Cl−Cl^-. 2Cl+2e−→2[Cl]−2Cl + 2e^- \rightarrow 2[Cl]^-

Step 3: Electrostatic attraction forms the bond. Mg2++2[ :Cl¨: ]−→MgCl2Mg^{2+} + 2[\, :\ddot{Cl}: \,]^- \rightarrow MgCl_2

Explanation:

This is an ionic bond where Magnesium achieves stability by losing its valence shell, and Chlorine achieves an octet by gaining an electron.

Problem 3:

Explain why PCl5PCl_5 is considered an exception to the Octet Rule.

Solution:

In Phosphorus Pentachloride (PCl5PCl_5):

  1. Phosphorus (PP) is the central atom with 55 valence electrons (3s23p33s^2 3p^3).
  2. It forms 55 single covalent bonds with 55 Chlorine atoms.
  3. The total number of electrons around PP in the bonded state is: 5 (original)+5 (shared from Cl)=10 electrons5 \text{ (original)} + 5 \text{ (shared from Cl)} = 10 \text{ electrons}

Explanation:

Since Phosphorus has 1010 electrons in its valence shell in PCl5PCl_5, it exceeds the octet (88 electrons). This is known as an 'Expanded Octet' and is possible due to the availability of empty 3d3d orbitals in Phosphorus.