University:
University of Phoenix
Course:
CHM 150 | General Chemistry I
Academic year:

2019
Views:

296

Pages:

14
Author:

Natasha Acevedo

Chemical Bonding - I Ionic Bond: This type of bond is formed between two dissimilar atoms (A and B). Atom A has one or more electrons more than its nearest inert gas configuration and has low ionization potential. Atom B has one or more electrons less than its nearest inert gas configuration and has high electron affinity. Atom A transfers one or more electrons to atom B, so both atoms achieve inert gas configuration. Atom A and B change into A⁺ and B⁻ respectively. Atom A (which loses electrons) is oxidized to A⁺, and atom B (which gains electrons) is reduced to B⁻. The ions A⁺ and B⁻ are held together by electrostatic forces of attraction. As a result, an ionic or electrovalent bond is formed. Ionic bonds are non-directional. Formation of Sodium Chloride: Na → 1s² 2s² 2p⁶ 3s¹ Sodium atom has one electron in its valency shell. Cl → 1s² 2s² 2p⁶ 3s² 3p⁵ Chlorine atom has 7 electrons in its valency shell. Na transfers its one electron to chlorine, forming Na⁺ and Cl⁻. Conditions for Formation of Ionic Bond: Low Ionization Energy: The atom should have low ionization energy to form a cation by losing one or more electrons. Example: Na → Na⁺ + e⁻ Electron Affinity: One atom should have high electron affinity, gaining electrons to form an anion. Example: Cl + e⁻ → Cl⁻ + 383.5 kcal/mol Lattice Energy: It is the energy released when oppositely charged ions attract to form one mole of a substance. It is also the energy required to separate ions in an ionic solid. Calculation of Lattice Energy: The lattice energy (U) of an ionic crystal is determined by coulombic attraction between the ions. These interactions include: Attractive interactions between ions of opposite charge. Repulsive interactions due to interpenetration of similarly charged electron clouds. 𝑃 . 𝐸 . ( 𝑎 𝑡 𝑡 𝑟 𝑎 𝑐 𝑡 𝑖 𝑜 𝑛 ) = − 𝑍 1 𝑍 2 𝑒 2 𝑟 P.E.(attraction)= r −Z 1 Z 2 e 2 𝑃 . 𝐸 . ( 𝑟 𝑒 𝑝 𝑢 𝑙 𝑠 𝑖 𝑜 𝑛 ) = 𝑏 𝑒 2 𝑟 𝑛 P.E.(repulsion)= r n be 2 The net potential energy (P.E.) is given by combining the above equations: 𝑃 . 𝐸 . = − 𝑍 1 𝑍 2 𝑒 2 𝑟 + 𝑏 𝑒 2 𝑟 𝑛 P.E.= r −Z 1 Z 2 e 2 + r n be 2 This is called the Born Equation. Application of Born Equation: The Born equation is used to calculate the lattice energy of ionic crystals. Example: In a NaCl crystal, each Na⁺ ion is surrounded by six Cl⁻ ions at a distance r, contributing to the attractive and repulsive forces. Sum of all terms gives the Madelung Constant (A), specific for each crystal structure. For NaCl, 𝐴 = 1.747558 A=1.747558 Conclusion: Lattice energy is directly proportional to the charge on cations and anions. Higher charges result in greater lattice energy and stability. Lattice energy and stability are proportional to the Madelung constant, depending on the coordination number and arrangement of ions in the crystal. Lattice energy is inversely proportional to interionic distance; smaller ions lead to higher lattice energy. Born-Haber Cycle: A cyclic process used to calculate the lattice energy of an ionic compound. Direct Process: Combination of atoms directly forms the compound, releasing energy. Alternative Process: Sublimation: Solid metal atoms are converted to gaseous atoms. Dissociation: Non-metal molecules are dissociated into atoms. Ionization: Metal atoms are ionized, forming cations and anions. All energy terms contribute to determining the lattice energy.