Chem Bonding I

• Chemical Bonding (I)
• 2024 Semester 1
• Ionic bonding


  Electrostatic attraction between oppositely charged ions
• Covalent bonding


  Electrostatic attraction between a shared pair of electrons and positively charged nuclei
• Metallic bonding


  Electrostatic attraction between a lattice of positive ions and delocalised electrons
• Ionic bond formation


  Atoms gain, lose or transfer electrons to obtain stable octet configuration
• Covalent bond formation


  Atoms share electrons to achieve stable octet configuration

  
• There are exceptions to the octet rule, e.g. NO2 has 7 electrons, SO2 has 10 electrons in the valence shell
• Dot-and-cross diagrams


  Represent electrons in 3D orbitals around the nucleus, not fixed orbits
• Only valence electrons are shown in dot-and-cross diagrams
• Types of chemical bonds

  • Ionic
  • Covalent
  • Metallic
  • Hydrogen bonding
  • Permanent dipole-permanent dipole
  • Instantaneous dipole-induced dipole
• Chemical bonds


  Electrostatic attractive forces between particles (atoms, ions or molecules) resulting in a lower energy arrangement
• Formation of chemical bonds involves redistribution of valence electrons

  
• Attraction between positive charges (cations or nuclei) and negative charges (anions or electrons) gives rise to different types of chemical bonding
• Strong forces of attraction

  Ionic, covalent, metallic bonds
• Weak intermolecular forces of attraction

  Hydrogen bonding, permanent dipole-permanent dipole, instantaneous dipole-induced dipole
• Comparison of bond strengths is only valid for molecules with similar number of electrons
• In a water molecule, covalent bonds exist between O and H atoms

  
• Hydrogen bonding exists between water molecules

  
• Ice floats in water due to hydrogen bonding
• Structures of crystalline solids

  • Giant ionic
  • Giant metallic
  • Giant covalent
• Comparison


  Between molecules of similar number of electrons
• Chemical Bonding (I) 2024 Semester 1
• Glass of water (H2O)


  • Covalent bonds exist between O and H atoms
  • Each O atom is covalently bonded to 2 H atoms
  • Hydrogen bonding exists between water molecules

  
• Formation of ionic compounds

  1. Electron transfer from valence shell of one atom (that prefers to lose electrons, usually metals) to valence shell of another (that prefers to gain electrons, usually non-metals)

  2. Ions produced have a stable, complete octet structure of electrons

  
• Compounds containing ionic bonds


  • NaCl
  • MgO
  • CaO
  • MgF2
• Ionic bonds


  • Electrostatic forces of attraction between oppositely charged ions
  • Non-directional
  • Strong bonds
• Steps to drawing 'dot-and-cross' diagram of an ionic compound

  1. Write chemical formula

  2. Write symbol of metal in square brackets

  3. Determine number of valence electrons lost from metal atom and reflect as charge on cation

  4. Write symbol of non-metal and use dots to represent valence electrons

  5. Determine number of electrons gained by non-metal atom to achieve octet and use crosses to represent

  6. Enclose non-metal in square brackets and reflect number of electrons gained as charge on anion

  7. Check numbers of cations/anions correspond to chemical formula
• Covalent bonds


  • Electrostatic attraction between shared pair of electrons and positively charged nuclei
  • Usually formed between non-metals
• Compounds containing covalent bonds

  • Elements: H2, O2, N2, Cl2, S8, P4, C (diamond)
  • Compounds: H2O, HCl, CH4, NH3, BF3, CO2, C2H4, CH3CO2H
  • Polyatomic ions: NH4+, CO32-, SO42-, NO3-
• 'Dot-and-cross' diagram


  • Shows arrangement of valence electrons around atoms in covalent compound
  • Number of bonds and where electrons come from are shown
  • Only dots and crosses allowed, no open dots
• Exceptions to octet rule

  • Odd number of electrons around atom
  • Less than 8 electrons in valence shell
  • More than 8 electrons in valence shell (only for elements in Period 3 and beyond)
• If the central atom is in Period 3 and beyond, it can have more than 8 electrons
• Anions


  Assign electrons transferred from the formation of cations as open dot (o)

  
• Ions


  Include the overall charge on the ion using square brackets
• In covalent molecules, the atoms usually acquire the stable 'octet' structure. This is referred to as 'Octet Rule'
• Covalent bonding


  Occurs when the valence orbitals of the two atoms overlap
• Normal covalent bond


  Two orbitals are both singly occupied with an electron of opposite spin to each other
• Dative covalent bond


  One orbital is filled with a pair of electrons while the other orbital is an empty orbital
• Covalent bond is strong. Each pair of shared electrons is attracted by both the positively charged nuclei involved in the bonding